I have a roll of vinyl, maybe 18" wide. I need about 15 feet of it cut down to 6, but I want the cut to be very straight and even. I have an x-acto knife and a 2’ carpenter’s square. Any tricks?
I’d try leaving it on the roll and using a pipe cutter, if doable. Or maybe a band-saw?
How straight? As in +/- 1 mm or +/- 1 cm? Is this vinyl for a floor and you are missing a strip?
My first suggestion is to just lay it in place and cut straight down through both layers of vinyl to make a perfect seam between the two pieces- the line in this case doesn’t need be straight but the two sides will mate perfectly.
If you need a perfectly straight line, buy a combination square, mark ever inch or two along the full length and then come back with a long straight edge (like a level level or ruler), align over the dots and razor blade it.