There is no way to get enough oil out of the pan to prevent it from contaminating your weld. Pull the pan, pressure wash it inside and out, and find the ends of the crack. Stop-drill the ends with about a 1/16" drill bit to keep the crack from spreading as it's heated, and braze the crack closed. By stopping up the vents you're going to be creating a bomb once the oil inside the pan gets hot enough to flash burn.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!