I have been sharpening my saw for 5-6 years now on the ground, sitting on it. I finally broke down and got a vise I am actually looking forward to sharpening my saw next time, lol.
I made a portable c-clamp vise with open cell foam stapled to the bottom of the noodled slab. It stays put and doesn't slide around on most surfaces. I use it mainly on either my table saw or the tailgate of the truck, both a pretty good height to be stable and comfy for sharpening.
I learned on a bench mounted vise, so it was hard to adjust to another method. I have seen guys hug the saw, cradled with legs and kneeling down and it looked impossible. You would need a fairly fresh file on hand.
I use a vise at home holding either chain itself or the bar. In the field I use a log or my tailboard (what's left of it) I'm liking that C clamp vise that MAF143 made.
I started with one of these: Then used my bench vise till I learned to square file. Then it was clamping the vise grips in the bench vise, which held each individual tooth, like this: Did that for many seasons till I bought a grinder. Seen a lot of guys use these with great success.