Buffy is likely somewhere in that ballpark.
We KNOW that Vampires are generally stronger than humans, but we also know that humans can fight them off, at least younger, more normal vampires.
We also know that the 'super soldier' types could beat multiple normal vamps, but were probably not going to individually beat the Angel's of the Buffyverse...
So yeah, I think being a vampire gives you a boost, but normal joe schmoe doesn't just become a Cap peer by becoming a vampire...but I think Angel and the other elite vamps probably get into the Cap ballpark, or slightly above, maybe as high as the 1 ton level (but no more than that)
As to Buffy's strength, she's more than 2 X as strong as the Angel and Spike types, as evidenced by the troll hammer incident as well as some of the comments from Angel et. al.
Some Angel feats:
Angel lifts a security gate in a museum that went on lock down....a Conan like feat IMO
Kicks this punching bag so hard that the chain snaps, and it flies to the other side of the room....peak human type showing at least
Angel rips out a Iron grate in the sewers....no indication it was rusted through, so another decent feat.
Slams guys with enough force to shatter the concrete wall....probably greater than peak human
Smashes a solid stone sculpture to pieces with his fist....at least peak human
Angel punches through 6 inches of Plexy Glass with no trouble....greater than peak human
Some Buffy feats
Buffy smashes through a huge chunk of brick wall.
2) Buffy slams this demons head so hard into the mooring post, it splinters to pieces! That is thick as a telephone pole.
3) Buffy jumps up fire escapes with a person in one hand.
4) Faith showing her Slayer strength by smashing through concrete while being heavily injured.
5) Buffy smashes Demon through a brick wall.
6) Buffy kicks another Slayer 50 feet away.
7) Buffy smashes a stone statue with a kick.
8) Buffy bends jail cell bars to get out the nerds when Willow was
coming for them
9) troll hammer lifting
10) in the episode Anne, she lifts a huge gate that the others couldn't budge...hard to quantify
11) she lifted a steel girder easily on a construction site, while two larger men struggled to carry one
Another Slayer with the Slayer powers is hit by a Semi Truck. No major Damage, just a headache afterwards.
All in all, it varied, but vampires ranged from very strong normal humans to probably up to about 1 ton (maybe a little more), and Buffy was probably somewhere in the 1-5 ton range, but hard to say where in that range.