The Old Model Vaquero is larger than either the Colt's Single Action or the New Vaquero. Whether you can stretch the holsters enough to handle the Old Model is a question you can only solve by trying it. Assuming the leather is vegetable-tanned cowhide and NOT soaked in oil, you can try wet-fitting the holster. Run tepid tap water over the leather inside and out for about 15-30 seconds, or until the leather is uniformly damp. Shake and rub off any beads of water to prevent watermarking the leather. Put the gun in a plastic baggie, wrapping it tightly around the gun. Work the gun into the holster. You may need to buy some hardwood dowel from the hardware store and round one end with a wood rasp and sandpaper. Buy a 7/8" diameter dowel, a 1/2" and a 3/8" one. The larger dowel is shoved down to the toe of the holster. The smaller ones are to widen the holster body close to the main seam.
Leave the bagged gun in the holster at least 2 hours. Then remove it and allow the leather to dry a few more hours. Put the gun back in , working it around until it fits. Do NOT allow the ejector rod to form a pocket in the body, but make a channel from the deepest point it rests in the holster up toward the top. Allow to dry overnight and keep trying the gun, without the baggie.
If this doesn't work, you will just have to buy a holster that fits the Old Vaquero. The larger holster will probably work okay with the Colt's.