We have done quite a bit of testing with vBackup 4.0.0 (and 4.0.1 pre-releases) and vSphere 6.0. The only major issues we have found is that VDDK 6.0 is very buggy and connections to ESXi 6.0 host servers require SSL verification. If you upgrade to vSphere 6.0 we suggest using VDDK 5.5.4. You will also need to install a CA-signed SSL certificate on your ESXi host server(s) to use any type of off-host integration utility (such as VDDK which is used by vBackup).
Hope this helps.
My apologies. Should have read VDDK 5.5.4 (5.5 update 4). Sorry for the confusion.
I have an esxi 6 host and had to use the older VDDK to get things running. It runs really good, no complaints. Getting 400mbits speed on a 1gbps link via ssh tho, im sure ill figure that out eventually.