If one wanted to approximate the flavor of beer in say PG/VG for mixing, I would suggest simply contacting your local homebrew outfit and getting some whole dried hops.
Beer is a surprisingly simple beverage to make and the strongest flavor, the "beer" flavor as it were, comes from hops.
I do NOT recommend hops pellets (ugh) but getting a packet of some quality hops. They are dried and are sold in silvery mylar vacuum-sealed packets.
Seriously. Stay away from pelletized hops. Look like rabbit food. No good.
Hops, good hops, pack a LOT of flavor in relatively small space, most of which is acid based, tho some oil. As you wouldn't be heating this in your PG/VG, I wouldn't be too worried about oil clogging up your atty.
Simply crush/bruise the dried hops between your fingers (a sniff and you'll smell 'beer') and drop into you PG/VG to soak for a bit.
Hops are bittering agents, so you'll probably want to experiment a bit with the soak time to make a formulation that is right for your taste.
Hope this was helpful
-K