Do you have UAC enabled on the DMZ VMs?
If yes - you have to use ".\administrator" or "domain\administrator".
All other accounts will not work with the combination VIX / UAC.
There is no other thing you have to setup as Veeam automativaly switches to VIX if network mode is not possible.
btw, it is possible to switch the default to VIX (https://www.veeam.com/kb1671
Usefull if you have an isolated backup network
If you have exactly
the same Problem as the dude in the blog - he has the solution /workaround
"As a workaround you can use a guest interaction proxy with Veeam 9 that is installed in the DMZ and bag your security guys to open the needed two ports between the backupserver and the guest interaction proxy. If this is not an option, please be careful when upgrading to VMware Tools 10.x
Coming from a call with Veeam to discuss this problem and it seems this is a known problem with VMware Tools 10.x. Veeam already opened a call covering this problem at VMware in FEBRUARY!!! but still there is no general solution available. A workaround is to downgrade VMware Tools (well, I already mentioned this possibility above) or wait for VMware Tools 10.1 where this issue is addressed. It seems that there is a hotfix available at VMware but hotfixes are published only in severe situations where there is no other way to get things work so it seems it's not that easy to get the patch.
VMware support told me to either bring a real good reason why they should give me the hotfix (probably it can cause more trouble than it can fix....) or wait for version 10.1 scheduled for October 2016."