Vigilant on 6" pipe?

Wel, no responsible person in the stove "biz" would ever advise reducing the size of your pipe. Likely it will result in a poor draft, and so it may be hard to strt a fire, difficult to keep the flue temp. up, and you may get smoke in the room when you open the doors or griddle among other things. All that said, I have seen it work, but really I wouldn't bother moving the stove knowing all that. Sell it, use that $ plus whatever you'd've spent moving it, and get an Oslo or something. That's what I would do, and I'd be happy as a skunk at a perfume convention.

I have seen vigilants that run on 6" flues but I have also seen several that didn't. Draft is house specific. It its a tall interior flue that discharges near the top of the roof with no major obstacles up wind of the stack it will probably work, but on rare occasions it could be cranky to start and you will need to run it on bypass longer. I would suggest reviewing the original owners manual to see if they comment on flue size. I expect that if you search for Vermont Castings Vigilant Operators Manual you may get lucky.

unlike some of their newer models the old vc vigilant was not tested or approved to run on 6" pipe

can you fit an ovalized 8" pipe down the chimney?

The old VC Vigilants will work on 6" stove pipe, but in that configuration you cannot run the stove as a 'fireplace'. That is, it won't draft well if being used with the doors open and a screen in front (bypass damper open). While it is true that the initial certification done for that stove used only 8" pipe, later models with identical flue pathways were approved for it, so it should not pose a functional issue provided you have the correct recommended 16' minimum flue height.