I use LogMeIn's Hamachi, but I mostly deal with Windows. Glance at
haguichi for Hamachi on those laptops or look into
remobo. It is my understanding that there is software available for those laptops that will let them connect to the MS Windows' Remote Desktop service as well as VNC software.
There are situations were the VPN tools cannot connect in a useful way. So I tend to install multiple ways to connect - mainly so that I can attempt to resolve any connectivity issues with my preferred method of connecting. I use TeamViewer and LogMeIn Free - which do not require a VPN connection. And MS's Remote Desktop Connection (RDC or RDP for short) and UltraVNC - both connect directly to LogMeIn Hamachi's static IP.
As I understand it, these VPN IP addresses are not able to be scanned for open ports from outside of the VPN network - so there is no issue with automated attempts at getting in via RDP or VNC and this
RDP vulnerability was a non-issue from outside of your LAN or VPN. Having static VPN IPs solves some issues with remote connectivity and you don't have to open/forward ports on routers. I personally like static IP addresses inside a LAN too, but that is just me.
You should set the Windows firewall on each VM (and the host computer) to restrict VNC and Remote Desktop connections to a list of static IP addresses within your VPN network. You can also add the IP range for your LAN in case there is an issue with one of the VPN tools. e.g. VM2 is not joining the VPN. You can remote into VM1 or the host computer and then "remote" from there into VM2 using its LAN IP address or computer name to see if you can resolve the VPN client issue.
For one group that I support, I setup a rather generic headless file server. On the desktop of that file server is a folder containing VNC shortcuts to each computer on their LAN. If I cannot remote into a particular computer, I can usually remote into the file server and "VNC" to the other computer to restart it. Then remote into the desired computer directly. At times, it is the file server that I cannot connect to, but if I can remote into any other computer inside that LAN, I can try to "remote" into the file server from there. But there are times when I just have to get into the car and go replace a PSU :-(
There will be a slight performance hit when using a VPN tool for remote control vs, a direct connection; but for most setups, you will not notice it. I'm active in LogMeIn's forum for Hamachi users and a few people see terrible lags on pretty decent connections to the net. I've never experienced such an issue.