If you ever need to replace the vanity, the new one can't have a smaller footprint than the current one. If that's not a concern, then it shouldn't be a problem.

Slightly less toe kick space so if you have giant toes then it’s a problem (just joking)….what kudzu said

It used to be normal to do it like that. But tile used to be more detailed and cut to fit the room. In some areas, like where I am, it's still done like that--vanity first. If you are using tile borders along the edges (like many historical-inspired tile jobs), or cove around the whole thing, it'll turn out better with the vanity installed first.

Tile guys like to do tile first because it makes their job easier. Vanity installers like to do vanities first for the same reason.