It's also entirely possible that they wanted to maintain flexibility to respond to Greg's response to the giraffes.
i.e., if it turned out Greg had anticipated them sending in summons and hadn't ordered the vampires to focus-fire Vaarsuvius to the point of ignoring all the others, and there was now (let's say) an enraged fiendish dire elephant bearing down on Elan, Vaarsuvius could have cast Stoneskin on Elan.
"And Vaarsuvius was hurt, but they, as well, can go to--"