I obtained a vanity bitcoin address (1MonkeyQWbMzny7BNTA66W9wv6KaCFFAB7) along with its private key. I would like to be able to use this as my main address.
Sites like BlockChain.info allow you to import the bitcoins with a private key, but is there something where I can enter the address and private key and be able to view how many bitcoins I have, and send them from THAT address (not import them to another one, like BlockChain.info does)?
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.
Pure technically, there are many wallets that enable you to import single private key. I personally like Electrum, which can be both desktop and mobile app. You can have multiple wallets inside Electrum, i. e. single-address wallet (which is your case), Legacy-HD, Segwit-HD, or even all of them if you like. If you work with one, it will not mix addresses with others. What you're trying to avoid is 'sweeping' the private key, which resends all funds from your vanity address into existing wallet and never reuses your (vanity) address again.
I don't recommend using bitcoin-core to accomplish this. Creating raw transactions manually is very inconvenient.
Apart from this, I have several other recommendations regarding your question. Don't use addresses/keys you obtained - generate vanity address by yourself - e. g. with this app.
Don't use online web wallets unless you have no other option. Blockchain.info may seem to be nice and good choice at first sight, but it's not necessary to give keys of your money to some internet company to look after it.
Vanity address may be cool, but use it only as a bitcoin contact card. For everyday transactions and payments resend the funds into Segwit HD wallet. It will significantly reduce fees and improve your privacy.
To check your balance, don't enter private key anywhere, just use blockchain explorer like blockchain.info or blocktrail and your bitcoin address.
