You do NOT have to use a 20A GFCI receptacle, a 15A will be just fine. Yes, you may run the vent fan off the load side of the GFCI receptacle as long as the circuit is dedicated to that bathroom. With a 48in vanity with a sink in the middle, place receptacles on either side of the sink, approximately 2in above the backsplash and at a location that does not interfere with any mirror you may be using. Only one of these needs to be a GFCI receptacle, the other can be a standard receptacle fed from the load side of the GFCI.

A bathroom circuit not shared with other room is permitted to serve both the receptacles and the lighting. The one thing I would avoid doing is putting the lighting (except for things that specifically require it) on the the load side of the GFCI (it's kind of annoying to lose the lighting if you trip the GFCI). With the exception of the few circuits required to be dedicated to receptacles (kitchens, laundry, and multiple bathrooms on the same receptacle circuit), there's no code reason to put lighting on a different circuit than the receptacles.

The information you provided (in 2 posts) does not provide a complete and accurate picture to correctly and safely advise you on how to rewire this. Losing the pictures you took is the big problem. Not tagging each wire based on its original usage is as much of a problem. I understand you not wanting to "get someone in to do this for you". You've certainly made a mess, now, that would raise that cost significantly. At this point I can only give you a very generalized suggestion. You need to carefully figure out exactly where every wire goes and where it is wired to on the other end. Start by putting tags on each wire and label each wire with a letter and number combination. Use a letter for each cable (or conduit) and a number for the wires within that cable. To help in consistency, use number "0" for the ground wire (it will probably be a bare wire in NM or UF cable, and will be insulated green in other case) in each cable, and "1" for the wire with white insulation. The remaining wires will be "2" and higher. So wire "2" from cable "A" would be labeled "A2". Be sure they are marked in a way that cannot fall off. Take new pictures of the setup to show where each cable and wire comes out in relation to each other. Trace where every wire goes. Double check that with an ohm meter that should show near zero ohms when the wire and meter is connected to others, and near infinite ohms when not connected at the other end. Be sure every wire going into a cable is in fact coming out of that cable at the other end to be sure there are no hidden junctions you have overlooked. Then list each of these cables and wires along with their destinations, as a list and post it that way. That can make it easier for people to help you that would otherwise "have to be there to see it". Well lit close up photos at each box would help (properly commented as to what box it is). An experienced electrician would be able to do this without all this chore based on his experience helping him recognize the situations that remain after your unwiring. But it would still cost more than if he were doing it from the original still-wired-up situation. One alternative is to put everything back as you had it before and see if everything still works as it did before. Hopefully you won't burn your house down or be killed by not getting it back the way it was correctly. Sorry to be so blunt. But you got yourself into this. And maybe better wording in your post will help, too.

Good point about using the fan alone! So, you would put the shower light on the same switch as the other Overhead recessed lights? Do you think we have enough lighting, and in the right place? Here's the plan: 6" recessed with lens- shower 6" recessed with lens- over tub 6" recessed- toilet area (34" wide space) 6" recessed - over central area, centered on vanity two pendants (~40 watts, not chosen yet) at vanity