I need to move a vent stack. I'm having a plumber come out to reroute it, but I need to cut it off to make room for a header.

1) Can I just cap the roof opening and leave the flashing? If so, what would I use to cap it off?

2) If I support the vent stack at the floor with riser clamps set on floor joists, can I cut the stack with a reciprocating saw?