How can I use a vanity address as my main address 
This is a bad idea.
Satoshi himself explained that for increased security and privacy you should use a new bitcoin address for every transaction.
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As an additional firewall, a new key pair should be used for each transaction
True that reusing addresses does harm your privacy and slightly harm your security, it is not advised.
But AFAIK, it still doesn't pose any risk to your bitcoin unless there are problems in your transaction signatures or ECDSA algorithm, or those on-chain gambling sites would have been gone.
Main risk i see is ones who use a site to get them. I just hate to trust others in creating a vanity address.. It is worth firing up a video card or two if you still have them to do it yourself then import into favorite wallet.
I would agree with others with not using it to hold your whole worth.