I'm a US citizen and Canadian permanent resident, living in Canada. My experience with Vanguard has been that they aren't willing to hold accounts for US citizens who are residents of Canada. The exception has been my 403b, which is technically held by a third party (even though it is Vanguard-branded). I didn't have a 401K with Vanguard, so I can't speak to that specifically, but I did have both a taxable account and a Roth IRA. I moved the taxable to Questrade and the Roth to TD Ameritrade, and I've been happy with both.
That said, I don't think that Vanguard would necessarily force you to liquidate it. When I asked about my Roth IRA, they said I just wouldn't be able to make purchases (in other words, it would be withdrawal only). This could change, though, and there have been situations where people got letters from US brokerages giving them X number of days to relocate their account or have it liquidated. Thus, I was interested in finding Canada-friendly brokerages.
As far as understanding the tax implications of rolling it over to an RRSP, I strongly recommend having a good crossborder tax person. Feel free to PM me if you're looking for a recommendation. I'm still doing my own financial planning, but I do have a tax person and that has been very helpful. He generally doesn't recommend rolling things over from an IRA / 401K / 403b to an RRSP because you typically lose some capital due to the early withdrawal penalty. Per him, it's usually more advantageous to just keep it as an IRA / 401K / 403b. YMMV.
TFSAs are generally not recommended for US investors because they aren't recognized by the US and require more work from a tax perspective. At the same time, he said once I have significant room (e.g. 20-30K) they may be worth it. Because it isn't recognized as a retirement account by the US, you would need to be mindful of PFICs here (unlike a RRSP).
And it's easy to stay invested in Vanguard. For my taxable account (at Questrade) it's in USDs and I just kept the same US ETFs. You don't want to invest in the Canada-domiciled equivalents outside of an RSP, though, as that would trigger PFIC filings. And if you leave your IRA / 401k with a Canada-friendly US broker, you can keep the same Vanguard ETFs. Oh, and if you have your holdings at Vanguard in mutual funds, you can just journal them over to ETFs before you move them (it just takes a phone call).
I have a website where I documented my experience in trying to figure this stuff out (see link in signature) so feel free to check that out. It's complicated, but it isn't too bad.
One more important thing -- if you have a Roth IRA, be sure to file an election form for it with the CRA with your first year's tax return. That way, it will be recognized (and tax free) in Canada as well.