There are several levels that unlock features similar to what you receive when you become a member (i.e. more open seats). Are these stackable with membership benefits?

Example: I have 14 open seats now (I think) and if I bought a membership does that mean I have 14+20? Does it work the same for vacations as well?

you`ll be able to create 20 player games, not 14+20. although as you lvl up you can create bigger games.
same for vacations, you`ll get 7/year.

http://warlight.net/Membership

I read through the membership section already, I suppose what I'm trying to say is it's unclear to me how my unlocked features correspond to what I get as a potential member...

You mentioned you can create 20 player games, but a level 1 member can do that. What about a level 29 player who bought a membership? Also, I currently have 3 vacations per year, would a membership give me 7 additional vacations, or just boost me up to 7 regardless of what I get now?

just boost you up to Membership levels, not additive. I'm a level 32 member and have the same features as a level 1 member.

Alright, thanks :D

actually you dont, a level 1 member has the new member benefits, you have the old ones from beign grandfathered account as well