I use them both as they each have their strengths and weaknesses.
My affection for both raster and vector began back in the CorelDraw 3 days where I began to use workflows that utilized both.
The question largely becomes one of benefits and constraints... and you mention an important one with lipsync.
Creating mouths (on a separate Level) in vector can allow for easy changes... and great automatic inbetweening as vectors allow for such while raster (and Toonz raster) will not.
Vector is resolution independent.
Raster is not.
Raster has MyPaint brushes.
Vector does not.
Toonz raster is a great mid point between those two with some of the strengths (and weaknesses) of both vector and raster.
But I find Toonz raster to be the go to level type for quickly laying down lines and filling them with color.
The list of strengths and weaknesses could go on and on... (I'd love to create a cheat sheet but it's better to just learn through experience especially as things will change and make that cheat sheet obsolete .)
Which one I prefer often depends on which one I am using. ;)
Currently, my favorite level Type is Toonz Raster and I find that a very happy medium and starting point... I love the overly generic line I can get with that.
But it's a rare drawing or animation from me that doesn't also have elements of vector and raster.
I'm closing in on a workflow where I convert between all three level types rapidly but I'm not quite there yet.
The goal (in this case at least ) is to be very fast. ;)