Hello.
I am a brand new user of MikroTik RB2001UAS-2HnD-IN, and am still getting a feel for the Routerboard OS, learning it’s ins and outs. I use a secure VPN through privateinternetaccess.com and I am having trouble understanding how to set up and properly use my VPN service on this router. I was also wondering if it would be possible once it was set up to be able to only use the VPN connection for certain IP addresses on my home network, it seems to slow down the online gaming on my sisters computer. I appreciate any help or suggestions and I am sorry if this has already been answered, I searched but did not find an answer to my question but I may not have used the right search words!
On your IP → Routes screen add a route that matches the IP of the server your sister is connecting too and set the gateway to your local modem. This should force traffic destined for that ip/range to go out that gateway.
I am still at a loss as to how to properly set this up, I am able to get through most of the steps in a tutorial I found but once I get to DNS and things like that I just don’t understand. I am coming from using standard consumer routing solutions where things like port forwarding and VPN are much easier to set up. I am like a little baby newb when it comes to understanding this router, the reason I got it is because I was sick of every top name consumer grade router I had ever owned dropping signal, rebooting, disconnecting, just all around failing. This has been a rock solid performer since the moment I set it up, wifi is flawless, ethernet connections never drop, my transfer speeds have improved across my whole network, especially my nas4free server. I am just looking for info or tutorials geared towards those with no real in-depth understanding of advanced networking or how to manually set this stuff up. I have searched and don’t seem to be using the right word combinations to get proper, easy to understand info about vpn setup and also simple port forwarding to be able to view my IP camera on my smart phone when not at home. .
Why would you like to use service like “privateinternetaccess.com” if you already have a Mikrotik router which is perfectly able to act as any kind of VPN server you can possibly imagine??? 
Configure dinamic dns client (find script on wiki.mikrotik.com) so you can access your router whenever and from wherever you want (if you do not have a fixed IP address), configure your router as PPTP, or L2TP VPN server, configure VPN client on your remote windows/linux computer and “voila”, you are in!!!
And when you are in (meaning - connected to your private network) you can do whatever you can do when you are connected localy.
enjoy
I setup something a while back and i am in the same boat… I posted about how to setup a VPN connection on the RouterBoard and then send traffic from a given device over the VPN connection… The post is here: http://www.tiernanotoole.ie/2012/09/24/MicroTik-RouterOS-VPN-Setup.html
I made some tweaks to the setup reciently,and will try and post an update with all the code later on.
As for why, think little sister in the same house, P2P and an ISP who don’t necessarily like P2P… this way, the ISP just sees an encrypted stream of traffic to a VPN provider, and the sister gets a working network connection…