I just started taking off my old vinyl floor (its the sheet type
probably put in as original flooring, house built in early 70s). It is
stuck really bad with black glue at the back. What is the best way to
take that gunk off......anyone done that? and what worked best.

If they installed it right the black glue is stuck to Luan plywood.

The luan covers the sub floor and its flaws.

pull up the luan and replace. its the proper procedure.....