I'll show you how to do this with the reference client.
First you need to import your newly obtained private key for your vanity address into your wallet. There is a great tutorial on how to achieve this here: https://en.bitcoin.it/wiki/How_to_import_private_keys_v7%2B. Once you do this, bitcoin-core will keep track of the balance for your newly obtained vanity address, so you don't have to do it manually.
Reusing addresses without a good reason behind that is not recommended. If you still insist on doing so, first you will need to get familiar with raw transactions and then take some time to read the answer to this question: https://stackoverflow.com/questions/23324442/is-there-a-way-to-send-funds-from-a-specific-bitcoin-address-in-a-wallet that explains how you can send funds from a specific address.
On top of that, you should be very careful when generating vanity addresses. These links will give you an idea: https://bitcointalk.org/index.php?topic=795023.0 and http://www.reddit.com/r/Bitcoin/comments/21foj9/funds_currently_being_stolen_from_vanity/. A reliable way to do this is by an unmodified version of a vanity address generator that would run in an offline computer that will never get online before it is low formatted.
