I have yet to read any Union Township historical references, and the internet kind of ignores Vauxhall, so I am afraid I can’t tell you anything about the Vauxhall post office. The architecture looks 1960s-1980s-ish (to my completely untrained eye), so it’s very likely not too historical anyway.
Here is what I can tell you: it’s a teeny smidgen east of a Whole Foods/ Best Buy/ Target shopping center, and its hours (as of December 2011) are:
M-F: 8:30am-5:00pm (lobby: 7:15-5:30)
Sat: 8:00am-12:00pm (lobby: 7:15-3:30)
Updates to hours can be found on the USPS website.
…Gosh I hope the post office doesn’t go completely out of business.