OK,
Well, as usual.......
I'll have a go!
I'd say that IF the hilt if definately secure and won't come off, then your best chance is to secure it by the hilt to a fixed position and use your hands to pull and wriggle the scabbard loose.
To that end, obviously a vice would damage it, so I'd try winding a cloth around it (tight) and then securing the other end of the cloth in a vice or nail it to something, then you can use both hands to spread the force over tha whole scabbard while you pull and wriggle it loose (hopefully)