I'm almost inclined to believe that there *was* no good, rational reason for bringing Vila along, but that Avon did it simply because, well, Vila's the one he best likes having along, not that he'd ever admit that.
I rather like that point of view. And certain Avon could come up with some good rational reasons to convince himself and the others that it wasn't just that he likes Vila's company.
If you look at Avon at the beginning of "Orbit," he's actually acting rather strangely,
Smiling a worrying amount for a start.
Leia