I and Geoff talked about this last time we got in touch with Skype.
If he goes after Trish, be sure she will not get out of the confrontation alive. The only chance for her is that Dante shows up quickly enough to save her beautiful ass and prevent Vergil from getting Sparda.
And then we'll have a huge dramatic moment where Dante has to choose side - the woman he feels he must protect or his brother, his other half?
I'd say he would choose Trish. When one looks at it closely, Dante is shield, the defense. Some see his behavior as lazy, I see it as him not being able to fulfill his purpose without someone he can stand in front of, someone to protect. So he stays on the side until demons threaten his own life at beginning of DMC3 (at this point his own life is the important one as he still hasn't had his epiphany), finds his true purpose in protecting Lady and humans, sits in his office until someone calls for help, doesn't actively pursue revenge until Trish is there to "activate" him, etc.
On the other side, Vergil is the sword, the offense, the one that requires someone in front of him, actively seeks out information, goes after power, dives in head first.
Shh, I am totally not using this opportunity to procrastinate... XD