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.