I was thinking about your situation, and the only real experience I have is with a VPN my company used to run we could use from home. That system was basically two different internet connections, with the broadband being the tube the VPN was routed through. The broadband connection had to be established and working before the VPN could connect.
If yours is similar or even close, maybe some thoughts would open some possibilities for a solution. I am trying to convert your complicated system to my simple system for comparison.
First, after you loose power and the entire system goes down, does the broadband connection come back normally? If it does, does the VNA wait until that connection is established before it attempts to re-establish its connection?
If the broadband does not re-establish, we need to get that working first. If it does re-establish, does it do so using IP addresses and network types (ie Private or Business) that are useable by the VNA? Does a location have any effect on the system?
If the VNA is trying to establish a connection to a static IP address to get its DHCP information, maybe there are some settings in the VNA that might be adjusted, assuming you have not already done that.
I will have to use a normal Network adapter for guidance, so let me know if you have other options. In the properties for the VNA, IPv4 properties, since it is a dynamic connection, you have the Obtain IP address automatically checked, along with the DNS address? You have nothing set for the Alternate Configuration tab?
If you go to Advanced Settings, have you changed anything there? If you have not, on the IP Settings tab, I was thinking specifying the static IP address of the DHCP server for the VNA connection might help. Possibly even putting in a Default Gateway would allow the connection to be established quicker. The Automatic metric option might be relevant, but your provider would have knowledge about that.
The DNS tab might help if the VNA had to search an extended time to find one, but not sure. The other options on that tab are over my head.
If you look at the Adapter properties, configure, Advanced tab, possibly something there would help allow the connection to establish. Not sure exactly which ones, but your provider might be able to give you guidance. Something that would allow for more leeway in establishing the connection, like buffer sizes or changing an Automatic negoiation to a known number, like link speed and duplex. I have not found one, but perhaps there is a place to increase the timeout for attempting the connection.
If you say the system works fine in XP, perhaps a comparison of different configuration options might be helpful.