I don't generally go into them personally, I just put them there as secondary choices. Triforce does indeed **** on gauntlet, but gauntlet is cheaper and provides more tankiness than the Triforce does. I only suggest Triforce while ahead for maximum deletion of squishies, but otherwise you'd want to be a bit more safe with some defensive stats. In my opinion. Also when you're really ahead who doesn't love a good ol' Black Cleaver, Last Whisper, Denting Blows attack. Either way I believe Triforce will always be the better item, but in the case that you aren't doing well then you'll need the Gauntlet.

On the note of the brutalizer, I only suggest getting it if you can buy it fully on the first back otherwise it can wait.

It's more optimal to get your jungle item first and THEN grab brutalizer instead of the other way around. If you're ahead you can still grab elder lizard first and after that judge whether you need to get more tanky or not.

Jungle items function as some sort of a GP10 right now which it's highly recommended to finish it ASAP.

Besides that, the bloodthirster and last whisper aren't really viable offensive items for Vi at all, and Triforce > Gauntlet.