I tend to use png. I then work out what the maximum size needed will be, and create them at that resolution. Disk space for images is cheap.
The only problem I have had is where I needed very large letters and it would have been a problem to load an entire alphabet in Mega-XXL. That's when I came up with
this method to get around the problem.
Another alternative is to have the same graphic at different sizes if it becomes too demanding on texture memory.
Having said all that, +1 for vector graphics. I don't know how practical it is in a game engine, but it would be nice to have for puzzle games etc.