By the end, you realise you've been watching two wildly different movies squeezed into one, and since Vaughn produced The Break-Up, worked on the script, and chose the director (the man responsible for the equally confused Down With Love), I think he's the one we should blame.
Vaughn and Aniston both have great, wounded eyes and, in The Break-Up's best moments, they connect beautifully. In a rom-com with any soul, they'd make quite a pair.