Windows Backup over VPN

I make my system BUs to an internal drive and then send the finished product to a computer in another building.

I don't know if mapping the remote drive, locally would work.
I kind of doubt that it would.

I started thinking about your question and looked around a little.
Some suggest that you can make an incremental BU to a VHD that is not local.
That would be the whole server BU and you could mount it to draw off any files that you need.
Here are some instructions:

Workaround to create incremental backup to network share

Windows Server built in tool Disk Management is able to create VHD/VHDX files, which can be displayed in File Explorer as a local drive. Then you can use the VHD to perform incremental backups, which is quicker and less disk consuming than full backups.

1. Type "diskmgmt.msc" in the search box or Run box and press Enter to launch Disk Management.

2. Click Action and select Create VHD.

3. Enter a network location and disk size. Then click OK.

4. Right click on the new created disk and choose Initialize Disk.

5. Right click on the unallocated space and choose New Simple Volume.

6. Follow the wizard to create a new partition.

7. Type "backup" in the search box and select Windows Server Backup.

8. Click Backup Schedule and follow the Backup Schedule Wizard to schedule an incremental backup task to the virtual drive.

Available Disks

Only in this way, will Windows Server Backup allow you to schedule incremental backup to net work share folder. You cannot set another scheduled backup but modify or stop the existing schedule backup. Rather than spend time creating one schedule backup, you can also create incremental backups directly to network share folder with backup software.

These came from an ad that was advertising a backup product:
http://www.backup-utility.com/windows-server/windows-server-backup-incremental-to-network-share-4348.html

They were selling a product offering an easier solution.
I don't know the product and have no idea as to whether or not it works, but have to give them credit for their instructions.

It will be slow (because it is over VPN). Can you limit the amount you backup? That is what I would try.

we are looking to be able to do a full backup followed by incremental.

VPN uses the slow side of the link up and down, so you will find it slow and need to allow time for it to complete. Just as a guess, it could take a weekend for a lot of data for a full backup.

What is the maximum Internet bandwidth at each end?  You can only backup data as fast as the slowest link.  Unless you have a LOT of bandwidth it will take a long time.

Say both sides have 100 mbps symmetrical Internet connections, both sites are relatively close to each other, and you have little other traffic.  You will be luckly to get  in the 70mpbs range for throughput, which means 7MB per second.  Now start doing the math to see how long it would take to do your backups based on how much data you have and how much changes during a day.

For the actual server BUs, I use Acronis. It is expensive, but I have used it to recover several times over the years. It gives us an option to automate the copying of the BU to another location.

Unfortunately, I still feel like I should have a SystemState BU on hand, in case anything goes wrong. So I also use backup to copy the system state twice a month.