This reduction in your download speeds will be because of how the VPN has an encryption process of your Internet Data. There isn't much you can do about this unfortunately as it does this to make sure all your work is secure and safe.
Also we wouldn't guarantee speeds from any 3rd party equipment. We would only guarantee speeds to the Virgin Hub itself.
I'm on Virgin M500 and when I'm using certain programs, I'm getting speeds of 2-3 megs compared to speeds of 10 -15 megs and more when iVPN is not on.
I use my Virgin hub in modem mode and using my Decos to spread the joy of wifi and I'm wired to my computer through the Deco.
Is that a typo? You're only getting 10-15 (Mb/s downstream?) without the VPN? If so then something is badly wrong either with the VM connection or the Deco setup and that needs to be looked into first before we think about the VPN.