I am remodeling a bathroom. I want to install a 36 " medicine cabinet but a vent stack is in the way. Can I 45 the vent stack out of the way and back to straight or must it stay straight?

If the vent is a sink vent then yes, you can offset the vent and return to the original vent as you propose... ;) You could even use a combination of 45s and 90s if it makes it easier, but make sure the piping pitches back toward the drain (see image), OK?

Mark

Mark is absolutely correct here. No worries. Proceed with your project and please post some pics of the finished product.