mining directly to exchange

I would think it is as bittrex doesn't change your address once generated, at least it hasn't in my 6 month experience. I would still suggest not to for the risk of them changing it at any given time, since you do not own the private key to that address. Sending VTC is practically free, so the smart thing would be to mine against your own wallet.

It should be safe. I did that in the past and there's never been a problem. No guarantees for other exchanges though.

Thanks, well we will se after new block :)