Wrong.
Whether a capital gain or loss is short term or long term is determined based on how long you have owned the particular shares you are selling. Unless you specify otherwise in advance, the law and the IRS assume you are selling the shares you have owned the longest. So each individual purchase that you make starts the capital gains/loss clock on those particular shares (often called a "lot" of shares...for example, the January 23, 2014 lot of VFINX...even if you only bought a few shares, it's still referred to as a "lot"). If you hold the shares for more than a year, then your sale is taxed as a long term capital gain or loss. If less than a year, then your sale is taxed as a short term capital gain or loss.
Rebalancing in a taxable account is considered a sale followed by a purchase, and the sale is subject to capital gain/loss taxes regardless of the fact that you subsequently purchased other investments. The purchase of the other investment starts a new capital gain/loss clock on those shares, regardless of how long you held the first investment. In other words, if you own VFINX shares for 10 years, then sell them to buy VTSMX shares and then sell those VTSMX shares six months later, you would owe long term capital gains/losses on the VFINX shares and the VTSMX share sale would be taxed as a short term gain or loss; it doesn't in any way "inherit" the 10 years from the VFINX holding.
Bottom line, you need to treat each set of shares that you purchase individually. (There is something called an average cost basis that you can use if you want to, in which case you do treat everything as one big lump, based on your total cost and the total shares. This is a simpler way to do things, and I believe mutual fund shares qualify for treatment this way. You may end up paying slightly higher taxes going this route.)
You might want to read the IRS topic:
http://www.irs.gov/taxtopics/tc409.htmlI do agree that if you have an emergency fund and are maxing out all your tax deferred options, then investing left over money into a taxable account is a good idea.
Good luck!