He did not take an oath to "protect and defend the Constitution of the United States." Only the President takes that oath. But that's completely beside the point anyway. You're not explaining what was unconstitutional about the vote in the first place. There's nothing in the Constitution prohibiting someone from voting on something in which they have a financial interest.
On the other hand, bills like the Glass Steagal Act itself are probably unconstitutional, since the Constitution reserves all powers to the individual states other than those explicitly granted to the federal government. In fact, most of the activities of the federal government are probably unconstitutional.
Since you're a Democrat, I doubt you have the foggiest idea what the Constitution actually says. Democrats tend to think it says whatever they want it to say.
The House has ethic rules which prohibit voting on matters in which one has a financial interest.