I'm a novice to League, silver 3, but I'm also a Ph.D. in chemistry and love looking for creative ways of playing the game. My builds are always wild, and many won't be as effective as the obvious builds, but if you're looking to shake up your gameplay and have a fun time then I highly recommend trying these out :)

Let's start by realizing that Vayne can theoretically build full tank, but still do massive damage because of her W since it doesn't rely on AD. Ok, now to the reality of why this is not common. I blame the items available, and specifically two aspects of them 1. only Wit's End provides both MR/armor AND attack speed and 2. every tank mythic item assumes close range combat (has immolation or dashes). Vayne needs some attack speed boost to do any damage, and any gold spent on immolation is completely wasted.

Other than identifying Wit's End as a key item, where does that leave us for this build? When there's no good option, let's try a couple :p

Everyone knows Rageblade works incredibly well with Vayne's W, effectively increasing the true damage by 33%. Starting by building this item (always cloak of agility before rageknife), we're guaranteed Vayne will be relatively powerful. The downside is that the critical chance on-hit damage is almost entirely wasted since Vayne will not build any more items with crit hit.

After building Rageblade it's smart to rush aegis of the legion since it's pretty gold efficient, and bonus armor/MR is more helpful early game. Next, build Wit's end, and then let's talk tank mythics.

Frostfire gauntlet seems like a solid choice for Vayne as she can effectively use its snowbind passive to create patches of slow in teamfights. However, the immolate ability is almost completely useless. Vayne will not actually be a full tank like Leona and so cannot rush the team and happily sit in the middle of the fray. Also, the mythic passive of increasing the champs size is specifically designed to help increase the immolation range, but not enough for Vayne to stay safe while burning eneimies, so it's also relatively wasted.

Instead, I think the tank mythic that better suits Vayne is Locket of the Iron Solari. It's not commonly used, but it better matches with Vayne's play style. Her goal is to stay alive as long as possible, and stay relatively far from the other teams assassins and fighters. Locket of the Iron Solari's passive has a long range and will increase her teammate's armor and MR by 9 when built. This may not sound like much, but ADCs have ~30 MR even at level 18, so an extra 9 MR prevents ~5% of incoming magic damage. Moreover, Locket's active shield ability is always helpful, and still very effective if Vayne is in the back lines.

Overall, Locket of the Iron Solari is the "best of the worst" tank mythic for Vayne since none others really fit.

I'm still not good at playing top, so I'd love for some feedback on how these builds work out! Please let me know in the comments below! I'll try this a couple of times in Bot lane, but I feel Vayne won't do enough damage to play this build as an "ADC". We'll just have to F around and find out :)