It's easier to cut vinyl flooring to the exact shape and size of your room when you make a template first. Staple sheets of paper felt to your subflooring and trim close to the wall. This line doesn't have to be exact. Transfer the contours of the wall onto the template using the inside of a square. Tape the template onto your new vinyl sheet goods and cut the true outline of the wall using the outside of the square. You should have a perfect fit.
For more on flooring, consider:
Quick and Easy Vinyl Flooring
Vinyl Flooring Installation (VIDEO)
8 Easy and Affordable Garage Floor Options