In the BB rules it says:
”Either way, at the end of the declared Action, but before actually passing, handing off, or scoring, the vampire must feed. If he is standing adjacent to one or more Thrall team-mates (standing, prone or stunned), then choose one to bite and make an Injury roll on the Thrall treating any casualty roll as Badly Hurt.” (Emphasis my own)
Since it says that the vampire must feed, that implies, to me, that there isn’t any choice. If you want the vampire sent off, you need to separate them from thralls.
On the the other hand, it wouldn’t surprise me if it were intended to be optional and Cyanide didn’t implement it properly.