I think that you're asking for way too much from a vintage steel anything, Dude. Much less a mountain bike. You may want to actually RIDE one a few times before you start ruling out options or imposing limits on what you'll accept.

The two things that you're asking for that show me that you're new to the mountain bike thing are your weight target and your dismissal of front suspension.

As for weight... do you know what owning a 20 pound mountain bike means? I can pretty much assure you that it isn't steel and it isn't cheap. You're basically asking for very lightweight steel road bike weights in a rugged mountain bike package. I don't see that happening... hell... a top shelf early 90's Trek 990 in full race trim weighed in at almost 24 pounds! And in the steel mountain bike world that bike is so light it's like riding a fart. It weighs nothing.

And front suspension... I suggest that you ride a mountain bike off road a few times before you jump out there and race one. There is a reason that front suspension is almost universal in mountain biking. You need to discover it. You'll have a very difficult time holding a line with a rigid fork and bumpy trails. Front suspension is a godsend on rough trails. If you ride very technical trails on a hardtail with a rigid fork... you'd better have extraordinary bike handling skills and catlike reflexes.

What you're asking for is possible, but you won't get it cheap or easy.