The vampire called to attention by Spike was thrown by him not being dead. Until he realized - what a fucking moron Gus was. This guy was a vamp, just like them. Angry that his other gang members had been played for fools he made his vamp face, baring fangs at Spike.
The smart thing to do would be to kill a Slayer while he could, but it took a lot for a dumb vamp to turn down a direct call to rumble. Ego, and all.
Mr. Vampy charged at Spike at a full lunge, aiming for his stomach. His plan was to topple him onto his back and then dust him - so easy!
Meanwhile, Gus was pleading with Faith. "I'll treat you's real good. Hey - I'll be YOUR slave, baby. Anything you want. You want diamonds? I'll getcha loads. You'll look so pretty in a diamond necklace...and earrings, and we'll make an entire coat of them for you, hell. Let's just kill your Slay friend, ditch the loser, and leave town! Say, you like Nebraska?"