The only reason to rollover a 401(k) to a new employer's program is a) they have great fund options with low expenses AND b) you perhaps
plan on making withdraws directly from the 401(k) when you hit 55. Since you're 26... I'm suspecting this is not relevant to you.
If you transfer to Vanguard, there's actually a pretty good chance it will be liquidated. Since this is a tax-advantaged account, there's no (immediate) taxable event, so it's no problem that it's liquidated, and once in Vanguard, you can buy the funds you prefer. Likewise, if they are not liquidated, you can do that - sell them and buy what you want.
When you talk about your "portfolio automatically rebalancing," what are you talking about? Do you mean in a new 401(k)? This would definitely rely on you having bought into their funds. In fact, I'm almost certain if you rollover to a new 401(k), the old funds will be liquidated in the process. So your new 401(k) will entirely consist of funds available from that program, and the rebalancing program they offer will sell/buy within those parameters.
Personally, I did rollovers using just a computer because I don't even like to talk on the phone, but yes, you can absolutely call Vanguard and they'll walk you through a rollover from a 401(k) into an IRA with them.
Hopefully that answers your questions. My personal preference is to move everything into Vanguard when that's an option, to hold just a very small selection of broad Vanguard "total" funds, and to rebalance myself once a year using a very simple Excel spreadsheet. Odds are, the funds in the new 401(k) are not as nice as Vanguard especially when you look at expense ratios!