There aren't a whole lot of obvious replacements for VMI if you are the Big South. You really need to find a football playing school which rules out most of the Atlantic Sun. The best option seems to be to try to get one of the Gulf South schools that would like to move up like North Alabama, West Georgia, or even, if the Big South could be so lucky, Valdosta State.
Right now the Big South has Liberty (wants to go FBS), Gardner-Webb, Presbyterian, Charleston Southern, Coastal Carolina and wildcard fb-only affiliate Monmouth for football. If I am the commissioner I would propose a truce with the A-Sun---urge the A-Sun to offer membership to those DII football schools in the Gulf South that better fit the A-Sun footprint and then offer an open invitation for football affiliatship in a combined A-Sun/Big South football league. It might look like this:
North Florida (starts up FCS program)
This alignment would be A-Sun heavy and probably technically be affiliated with the A-Sun but right now the Big South programs are up and going at FCS and the others need a conference structure to join in order to come to DI-FCS.