The old stand-by was a sling psychrometer which is just two thermometers. One has a small sock on it that you keep damp,then you twirl the unit around in the air,look at the wet bulb vs dry bulb temperature and look up the result on a chart that will give you a vapor pressure number.
I have a copy of the chart if it will help you,but after using the psychrometer and a digital relative humidity gauge I can tell you thet don't always match up the way they should.
Keith

Some info I found on the Bullit.
http://www.yellowbullet.com/forum/showthread.php?t=388229

Bill I send my air box back in and had VP put in it because they said it was better to dial the Jr with.. they was right..