I find this hard to believe, but these pages regarding vivo and vinco confirm this to be the case. This also seems to be confirmed on this website. I cannot link directly to the words vivo and vinco, so you would have to search for them yourself on the site.
Also, under the definition of victurus it just says "future participle of vinco", and nothing about vivo.
The same goes for staturus, which is the future part of both sisto and sto.
Sebastian Koppehel said the important bits but I want to provide some background for people who like that sort of thing (me). Don't let this confuse you if you don't find it helpful—it can be a lot when you just want to focus on learning Classical Latin.
The future participle of vincō being vīctūrus, while not synchronically regular in Latin, is unsurprising: there are a lot of verbs that have a so-called nasal infix in the present that lose it in other tenses. This is a relic of a Proto-Indo-European mechanism for forming an imperfective ('present') tense out of a perfective verb stem, which is no longer productive in Latin but still somewhat prominent (though not as prominent as it is in Greek).
Other examples include fundō/fūdī/fūsum 'pour out', relinquō/relīquī/relictum 'abandon', rumpō/rūpī/ruptus 'break'; in other cases the nasal infix is unexpectedly kept in non-present tenses some of the time (pingō/pīnxī/pictus 'paint') or all of the time (iungō/iūnxī/iūnctum 'join'), so this is a matter of studying your principal parts.
The future participle of vīvō being vīctūrus is part of a pattern where a present stem ending in -Vv- (where V is any vowel) alternates with a supine (which is what the future participle is built on) having -Vct-. The examples here are further complicated by intervocalic [w] often having been lost at an earlier stage, but they include: Old Latin fīvō/fīctum 'fix', fluō/flūctum (more commonly flūxum) 'flow', struō/strūctum 'arrange'.
This alternation originally seems to have been a consequence of the different outcomes of Proto-Indo-European voiced labiovelars depending on whether they were followed by a vowel or a consonant, but it also shows up by analogy in a lot of verbs where the root didn't originally end in a labiovelar, including vīvō.
Sistō and stō having the same supine isn't that surprising: they're built on the same verb root.
Reduplication, where you take the first consonant (or onset cluster of consonants) of a verb and repeat it at the start of the verb followed by some vowel, was a common occurrence in Proto-Indo-European, with e‑reduplication (where the vowel is an e) in particular being famous for being how perfects were formed (traces of which linger in Latin: pellō/pepulī 'drive out', pangō/pepigī 'fix' (supine pactum!), currō/cucurrī 'run'; in Greek the mechanism remained productive).
What's going on here isn't that, but i‑reduplication, which mainly seems to have formed causatives: stō means 'stand', sistō < earlier *stistō 'cause to stand'. In PIE this particular reduplication only appears to have existed in the present, which is why they're often called i-reduplicated presents, and also why in the daughter languages this reduplication is often not present outside of the present.
They're (again) more prominent in Greek, but other Latin examples include: bibō 'drink' (cf. non-reduplicated πίνω in Greek, with a nasal infix), sīdō < *sisdō (= Greek ἵζω) 'sit down' (sedeō 'be sitting' is a stative built on the same root), gignō 'bring forth', serō 'sow' (the -r- is a rhotacised older -s-). Of these, bibō and sīdō are the only ones that have carried their reduplication through into the other tenses, but since the others don't have an extant unreduplicated present companion the way sistō/stō does, this can be a harder pattern to spot.
If you look up vivo and vinco in a dictionary, you will see that both have the same supine stem: victum. Thus they share not only the future participle, but also the two supines and the perfect participle. It is my subjective impression that victurus, when you actually encounter it, is usually a form of vivo, whereas the perfect participle victus from vivo is rarely found. But one could certainly construct examples for it.
There is also the nice wordplay (if one can call it that): nisi victorem victurum neminem (only the winner shall live; Cic. Phil. V, 21)
This also extends to the verbs convivo and convinco by the way.
online-latin-dictionary.com does not appear to be a particularly good website. For exploring declensions, I recommend Wiktionary. It duly mentions both meanings of victurus. (For actually looking words up, Collatinus is preferrable because it allows access to several older, but still very good scholarly dictionaries. It clearly beats Wiktionary in this regard.)
The stem of the future present participle when -ūrus is removed is usually (although not always) identical to the stem of the perfect passive participle and supine.
There might have been a difference of vowel length (at least originally) between the identically spelled forms of these two verbs: vinco forming victūrus with short i and vīvo forming vīctūrus with a long ī.
Although there seems to be little or no surviving evidence in the Romance languages of short i in vict- words from vinco, the etymology of vinco suggests that the vowel in the participle forms should be short: compare rumpo, rūpī, ruptus.
There is also apparently an Oscan form spelled víkturrai which Michel de Vaan says represents a borrowing of Latin Victoria (in the dative singular). Oscan has a separate letter transcribed í that represented a short i sound. While there isn't a guarantee that Victoria and the participle forms of vinco had the same vowel length, the name is related to the verb.
The Latin Language – a historical outline of its sounds, inflections, and syntax, by Charles E. Bennett, does support a short vowel in vict- for forms and derivatives of vinco, although noting that some prior sources designated a long vowel on the basis of some evidence:
victor, victus, victōria, etc.: ī Lewis, on the basis of repeated inscriptional markings, such as
vIctor, CIL. vi. 10056; 10115; 1058;vIctorinvs, vi. 1058;vIctoriam, vi. 2086;invíctae, vi. 353. But with a single exception no one of these inscriptions can be shown to antedate the third century a.d.; and I quite agree with Christiansen (de Apicibus et I longis, p. 49) in the view that in the classical period the i was short; later, apparently, it was lengthened.
(https://alatius.com/latin/bennetthidden.html)
On the other hand, "The cycle without containment: Latin perfect stems", by Donca Steriade, 2012, describes vinco as having a perfect passive participle stem vīct- with a long vowel, attributing the length to analogical extension from the long vowel in the perfect form vīcī (page 44) This sources does not describe the evidence for ī in vīct-, or discuss dates, so possibly it could be harmonized with Bennett's suggestion quoted above of a post-classical lengthening.
