Bent rear mech hanger

Ok, have a mid-90's steel frame (fillet-brazed Deda tubing) and having spent at least an hour trying to get the gears indexed (Campag {barely used} Chorus crank to {used} Xenon rear mech, 10 speed, brand new Campag chain) it looks like the rear mech isn't quite true - and the culprit looks like the hanger. Which is part of the rear dropout, so not detachable.
So... can I (very, very, gently) try and cold-set it straight? Have a good-sized Record vice, so am prepared to have a go. Welcome any advice!

Steel should be fine (not that you have a choice), but only bend the hanger with a wheel or axle fitted to prevent it bending where you don't want it to.

Doing it by eye is just as likely to make things worse (but a pair of pliers will be adequate to get you home), as improve the situation though. If you don't have access to a hanger alignment tool, then you need a second rear wheel.

With one wheel in the frame, screw the other into the hanger and fit a track nut on the back of the hanger: you now have both a gauge and a hanger straightenerer.

You might need to remove the freewheel/cassette or flip the wheel over for clearance.

Take it to a reputable bike shop and ask them to straighten it. There's a tool that makes it a five minute job in an experienced hand.

With one wheel in the frame, screw the other into the hanger and fit a track nut on the back of the hanger: you now have both a gauge and a hanger straightenerer.

This is clever clever tho!

To both Scilly.Suffolk and Howard, really appreciate the suggestions. As I've a. spare old rear wheels and b. no cash, I'll try the 'wheel-o-truer' approach.
Cheers

Right. Job done! Fantastic. Everything's indexed, everything appears (reasonably) true. The 'wheel-o-truer' approach is great, as I could measure off around the parallel rims to fine-tune the flexing. Will take it for a proving run tomorrow and see how we do.

I did mine with a large adjustable spanner and swinging on it when I was a kid. Impetuous youth...

Thumbs-up!

I forgot to mention, if you use the same points on both wheels (or the wheel and alignment tool) through 360 degrees, then it doesn't matter if the wheel(s) aren't true.

Was tempted to, before stumbling across this site. Only thing that stopped me was the cost of having to buy a big enough spanner...