Did you see my post in your other thread? If you come to get some of mine, now is fine- it's growing outside anyhow. If you already bought some or live too far, I'd think wherever you got it from, so long as they've hardened it off, you can plant now. It's very hardy. It'll take a while before it really takes off for you. Mine was transplanted from PA, and it did the typical 3 yr sleep, creep leap thing. I would just be sure where you're putting it is bordered somehow that it can't cross- when it gets going, it likes to spread! It's a very nice cover for certain conditions, though.

I grow a lot of vinca and it usually takes a while to show that it is doing well. If I plant a nice plant it just kind of sits there for a couple of years and then takes off. I would say just doo your best and relax. I would not fertilize til next year. the compost is enough for now and put some more on in fall. Vinca is a plant that is good for drier, poor soil so don't worry too much, just give it time.

Yes more than likely both of those will choke out your existing plants. At the very least you will be constantly working to keep them under control. I agree that Asarum is a beautiful and less aggressive choice. Arum would work well too. Particularly Arum Italicum.

I'm hardly an expert on transplanting them to the ground, since I must keep mine in the pots, but in general it used to be said that saw dust may rob soil of nitrogen as the bacteria trying to digest that saw dust will rob nitrogen from the soil instead , and if used supplemental nitrogen in the form of some chemical fertilizer is recommended. I'd guess it's fine to mulch with raw wood chips , but I'd not mix them or use as ammendments unless they've been composted, for the same reason as with using saw dust ! A well drained loamy mix should be sufficient I'd guess, with little in the way of extra ammendments. Since magnesium defficiency with some plantings may be a problem you might add some of that in some form as an ammendment, though just a top dressing would be helpful!