I would absolutely not concern yourself with going full-on 9.2 channel system. Stick with 5 channel and a pair of subs, properly set up. I'm going to assume you are OK with used gear, so my recommendations are taking that into consideration.
The Vista is really not a tight sonic match to the the "Motion" series of Martin-Logan speakers. I would try to sell the C2 and get a used Motif or Stage. The surrounds are not as critical of a match, but I would still try to stick with ML. If you can find used Tickets for a good price that would be a great way to go, but all of the ML product will be voiced to sound as much like Electrostats as possible.
I'm going to assume that will eat up ~$1000 of your budget. That leaves you another $3k for a pair of subs, a multichannel amp, and a turntable. Lets assume you spend $500 on a good turntable/cartridge, that gets you down to $2500. Some really good multichannel amps can be had for $1000-1500 all day. Some really good powered subwoofers can be had for $500 a piece.