He will not guarantee any such vote. However, he might hold one if the EU wanted significantly more powers, which he thinks is "unlikely."
But when you look at the detail of what he is promising it is interesting to say the least. The current coalition government has already passed a law putting on a referendum lock which means that power cannot be passed from Westminster to the EU without a referendum on those proposals.
Miliband appears to want to replace this with a different type of referendum lock whereby you have to have an in-out referendum if the EU wants more powers.
This appears to be a replay of a trick Tony Blair and Nick Clegg tried nearly a decade ago, particularly at the time of what was called the constitutional treaty (later known as the Lisbon Treaty). They know they could not win a vote on the treaty, despite having promised to hold one, but thought they could win an "In or Out" referendum vote, so they offered that instead.
If that is Ed Miliband's game, and he tried this policy as Prime Minister, I think there is a real danger that he might fall foul of the law of unintended consequences.
Kevin Beaty, number three on the Conservative ticket for the North West of England in the European elections has some thoughts on the subject which you can read here.
David Cameron also has some thoughts on the issue and he writes as follows: