You will still get charged, but only after the initial monthly bill - not at sign-up.
Having moved from PlusNet, they did cause a few problems in that the complained that Vodafone didn't tell them about moving the broadband (which Vodafone did). A letter I received did seem to indicate that PlusNet wanted to take broadband back, but a quick online chat soon sorted that out - plus a refund for charging for one month of non-use...
Its been going very well with Vodafone. Including the price of rental plus internet is the correct thing to do.
Unlink PlusNet, you can also pay before the actual payment date, and it appears that you can pay more than needed too (that will be tested early next month
Now with Vodafone
Working on BallZ #GO!