I have a vanity top that is in very good shape no water damage at all. Do I need to build a new one or can I just install the new Formica on top of the old one? And do I need to do anything to the old one before I install the new Formica?

As long as the existing laminate is very well adhered to the countertop you can put new over old. Sand the old with 120-150 grit paper, clean with mineral spirits, use contact cement-be careful if it is solvent based and turn off any pilot lights and open windows-coat the new laminate, let both dry so they don't tack, put brown paper on the countertop, lay the new laminate over the paper and starting at one end, slowly remove the paper, pressing down with a carpeted weight. Use a rubber mallet to further stick the two together. Finally, use a heavy roller to make sure they are stuck together, rout the edge or use a smooth file. Good luck.

I agree with ma0641 but you could use old slats from mini blinds to put under the laminate and pull them out as you smooth and roll the new laminate top.

And I use wooden dowel rods for spacers.

Sanding can be most any grit, as long as there is something for the new adhesives to stick to.. using a 2X4 to smooth the connection works well also.

If you are doing the edges as well,do the edge first making the top of the edging even with the old laminate,then applying the top,you will overlap the sides making for a watertight edge instead of doing it the other way which allows water to go between the top and edges.