Can I use Microsoft Paint for drawings?
There is no legal restriction on what software you use to produce the drawings initially. As long as the final product complies with the requirements of 37 CFR 1.84 (which basically requires clear line drawings), you should be fine.
Moreover, there is no requirement for vector drawings. While these are moderately common in practice (since there are a lot of technical benefits in preparing drawings in this manner), any line drawing would be fine: even scanned copies of hand-drawn images are ok.
As an aside, while there is no restriction on using Microsoft Paint, there are plenty of free (and paid) software options which you can use that will make your job a lot easier. I use Inkscape personally, but as daniel noted, Software Recommendations has a bunch of questions on this point for whatever your preferred OS is.