It can't be a bubble. Bubbles are what happen when people speculate that prices will remain high when all evidence suggests that prices must fall at some point. Basically a bubble is what happens when people's expectations are irrationally high. For the bubble to exist, people must be buying them with resale value in mind. This can not really be the case in the market for vintage bikes. I don't think that many people are buying vintage bikes because it's a cash cow.
Perhaps demand has risen recently due to things like gas prices, changes in tastes and values. The cost of making bikes has risen recently due to the rising prices of metals and energy. Also don't forget that the supply of vintage lugged steel frames will always dwindle due to the speed at which these things get hacked up to become fixies or end up in the dump.
Finally, given the fact that making a good lugged steel frame is a skill that is going the way of the dodo, and is highly labour intensive, we should expect that the prices of these frames would increase relative to other bikes as long as there are mass produced cheap alternatives.
Allan