I currently have a vanity with a built in light and a built in outlet that is turned on and off by a light switch near the door. I want to remove the vanity, install a GFCI outlet and wire in two new lighting fixtures.

I recently read that a GFCI outlet should not be controlled by a switch, but I couldn't figure out why... is that a bad idea? Currently, it appears it's my only solution - so that turning on and off the switch near the door will cut power to the new lights AND the GFCI switch. I'm OK with that as long as it's safe.

It's safe , but it's not legal if the circuit is shared with other rooms, which from the sound of it it probably is.

Can you remove the single gang switch box and install a 2 gang old work box and wire the switch parallel with the gfci?

So if it's safe, why wouldn't it be legal? Just curious...

The issue that Stan mentions is not legal is that a outlet circuit for a bathroom is to be dedicated for bathroom outlets, and not lighting.

Once 20 amp circuit can feed one or more outlets in one or more bathrooms.

One 20 amp circuit can feed outlets in one bathroom, and the lights and fan in that one bathroom. No other lights or outlets can be on that circuit, or vice versa.

As far as a switch controlling a GFI, I see no problem with that. I also don't see any reason for it, if you wire it properly you can have constant power at the GFI leaving the switch to control only the lights.