How T-slots and V-slots differ comes down to their shape on the outside. They both use T-slot for securing but the v- slot has the advantage of being able to be used as a rail for slot type wheels. 

The difference between T-slot and V-slot profiles has nothing to do with T-nuts or brackets—those components will work in either profile variation. 

When looking at used linear aluminum extrusion it’s import to be able to identify what it is. I’m sure you could use v-slot in a regular build but if you intend to have moving parts you need to have v-slot.

Alot of times it wont matter because using v-slot as a rail is not the best system it’s just the cheapest. attaching a linear guide rail with sliding blocks is a better way and more accurate but more expensive. So if you purchased a extrusion that doesn’t have a V-slot just upgrade the rail system