The vest holster idea here is very interesting.
I thought it would be fun to post pics of the Old Old Model Russian Hardin used to kill Deputy Webb in Comanche, TX, along with the holster and belt. I don't believe the belt has ever been photographed. Presumably, the holster was sewn onto the "loop" on the belt. It's possible it got broken in the aftermath of the shooting when Sheriff Carnes was trying to hand the gun back to Hardin when he let him go to escape a growing mob just after the shooting.
I've been trying to imagine if this gun could have been in a vest holster as the OP has shown here, but I don't think this particular gun would have been.
Also pictured is a pic from the graphic novel, Lost Cause by Jack Jackson. This drawing seems to be an accurate depiction of how it might have been that day.
As to how Hardin drew his guns. With this particular holster, the gun would've been drawn like a calvalry draw, with the grip of the gun facing forward. So, I suspect that is how Hardin would have drawn using the OP's vest idea, versus a cross draw. But, I'm no expert! Just a thought.
Hope you enjoy the pictures. I did the pic with that particular bourbon bottle because I thought I remembered there being a tie between Hardin and Knob Creek, but of course I can't find that now. I thought it was a fun pic to take anyway.
Lynn