Expand your search to include builders, gc's and licensed carpenters. Depending on where you live, there's a good chance the local window-specific companies are set up to maximize efficiency on the largest market segment - which in many areas would be vinyl windows.

A good builder or gc will be able to source and install aluminum windows that are more in-line with your mid century style, though.

Thanks okmrazor!

I was hoping to avoid going the general contractor route since I can already see the $$$ adding up.

I contacted a local glass company but given the age of the windows they can't retrofit it with dual pane.