My vertical slide is a Myford double-swivel with a vice a lot like the one in the bottom pic. I bought a Warco baseplate with my WM250V and modified it with 2 M8 tappings for the Myford slide. Using the T-slots in the WM250V crossslide, I have quite a range of positions where I can fit it, so I can *usually* find a convenient way to do the job within the 115mm. movement range of the crossslide. I'd find it very limiting to have to mount it in a fixed position such as the toolpost stud.
I'd thought about fitting tenons to locate it square to the spindle axis, but it's very easy to manage without – with the T-nuts loose, you run the saddle to push the vice jaws up to the face of the chuck jaws, with these latter as open as you can set them and still touch the top face of the vice, check for good contact and then carefully tighten the T-nut screws. Your vice will be square to the spindle axis with a thou or two max.
I use this arrangement a lot, and recently I've got lazy and taken to leaving it in place – with the swivel slide swung aside out of the way – even whilst doing ordinary turning.
Because this works so well, and I'm short of space, my plans to buy a mill keep getting put back.

Edited By Mick B1 on 02/02/2020 19:34:49