Although I'm not sure it was ever answered, there was a very similar question asked near the bottom of this thread:
viewtopic.php?f=8&t=44&start=0&sid=4722be60f8ba6c8e0840b8151fb44748#p137I would assume almost any sealed lead acid battery would work with Vegabond II. The chargers for SLA batteries generally look for a voltage delta to know when to stop charging, not how many Ah it has put in a battery. As long as you get a 12V SLA battery, I'd say you'd have little problems. Of course, the battery needs to have the capability of supplying enough surge current for a flash, which could probably be up to 50-60 amps, so you wouldn't want too small of a battery...
Maybe Paul will share some insight.