1. - Some lubricants are made just for speedcubing. One that probably lots of people consider the best is Lubix. There are also other stores, like the one on this website, that sell their own lubricants. The only one that I've tried is Lubix, and it worked well for me. 100% silicone spray is also one that is used for cubes.
2. - Vaseline will probably dissolve the plastic of the cube. This can help it turn smoother or faster after you first use it. However, it will eventually make the cube get worse. I wouldn't advise using it.
Before you lubricate a cube, it is better to use it for a while first. This will help smooth out any rough parts on the plastic. Also, it will help the puzzle turn better once it is lubricated. This is because you are applying the lubricant to smooth plastic instead of rough plastic.