Do you control the private keys?, if you don't then it is not secure.
Not secure like not secure as in a bank? Because I'm kinda looking for a bitcoin bank that can store the coins for me.
With multisig vault you control the private key though, it's 2 out of 3 needed and coinbase only carries one.
If you can guarantee them not having the other 2 keys, then it would be secure, if coinbase fails one day and the site goes off line or coins get seized they don't have access to that wallet (make sure the keys are bitcoin private keys not just a "vault private key"), as long as you have the two private keys you can then go make your own transaction to clear the wallet. If the private keys are generated on their servers, which I think they are then they could have access to them which then makes it insecure.
Its better you keep coin in a paper wallet. Share the private keys for that address to all parties including you and ask them to keep it in a bank vault. Never trust any online wallet for longer term and with big amount.
While this is partially true, remember a site or program needs to create these keys for you, if its connected to the internet someone else can always have the key. Generating the private key / address on a computer not connected to the internet and that won't ever be is the safest way to make sure the keys are truly private.
