Bernie is an unelectable Socialist (self-described) in the general election. Bernie has been an independent politician for the last 40 years and I can't grasp how he can be eligible for the Democratic Nomination since (as best I can tell) he has never been a member of the party.
Vermont has no party registration, so there was never a possible way for Sanders to really become a member of the democratic party. He caucuses with the Democrats in the senate. He typically votes with democratic leadership on important bills.
Leahy calls himself a Democrat and is a Democrat for all intents and purposes.
I haven't heard anything about Howard Dean, another Vermont Democrat, having issues back in 2004.
But, to go back to what you're saying, Bernie can effectively call himself anything he wants because there's no party registration. If it wasn't an issue 11 years ago for Dean, it shouldn't be now.