Useless? Not strictly. Worthwhile, hardly.
As to your concerns about the vampire. The reason why it is easier to defeat when it is using rats (Rats against a 12th level fighter?) as mooks, is that the Fighter has spent eight feats to learn how to deal with this situation. If you think of the synergy between WWA/Great Cleave as a virtual feat, one is hard pressed to think of another feat that has 8 feat and 3 ability score prerequisites. One extra attack is sort of like leaving a penny tip.
"Limiting cleave attempts to one per target per round is a fair way to prevent cheesiness, while still retaining GC's usefulness against hordes of mooks."
Actually it makes the combination weaker, if only because the use of Great Cleave at high levels (10th and above) will function better for the Fighter (and equally well for the Barbarian, Ranger, etc.)
Compare:
Icebear's system (one cleave per opponent): Great Cleave only: Attack first mook, drop it. Cleave all the way around the circle and finish with the Leader. Leader has been attacked once at the highest Attack Bonus. Finish off iterative attacks against the leader.
Total 3 attacks on the leader, one on everyone else. 11 Attacks.
Result: 8 Attacks at the highest Attack bonus, 2 iterative attacks.
With WWA/Great Cleave: WWA First Mook, drop him, continute WWA attack until finished. Get Cleave on Leader.
Total 2 Attacks on the leader, one on everyone else.
Result: 9 Attacks at highest BAB. A net loss of one attack by using WWA in combination with Great Cleave
A fighter can acquire WWA at level 6 at the earliest, and by Level 11 Great Cleave has roughly equalled it in power. By level 16 WWA will be completely useless in these situations because Great Cleave on its own will give you two more attacks. The feat with the fewer prerequisites has more long-term benefit.
Core Rules System: Great Cleave only: same as above.
Total: 3 attacks on the leader, one on everyone else. 10 attacks.
Result: 8 Attacks at the highest Attack bonus, 2 iterative attacks.
WWA/Great Cleave:
Total: 7 Attacks on the leader, one on everyone else. 14 attacks.
Result: 14 attacks at the highest attack bonus. An increase of 4 attacks over Great Cleave only at level 11, or 3 attacks at level 16.
Hardly an exponential increase in attacks at the levels where the combination is possible, but significant enough to justify the additional 5 feats rather than just taking Great Cleave.