The first thing to do is to quantify your needs and resources - particularly the CPU's in the available systems, and what the VPN is required for.
I have several offices which I connect using VPNs on an ASUS AC1900 dual band gigabit router. I use a similar router with VPN at home, and do heavy backups over it. (I do note that in both cases I'm only encrypting traffic to key destinations, but that would be the majority of traffic) - as if your needs are not extremely high, using a decent router is the way to go - Consensus is you should be able to get > 40 megabits over VPN from an ASUS RT-AC68 or better.
(The statement that " even the most powerful routers don't have enough CPU to encrypt and decrypt the data from multiple devices on your network." is patently incorrect - there are any number of x86 boxes with multiple interfaces configured as routers, and they are more then up to pushing anything a typical home environment could throw at it - especially if they have AES-NI on the chip - but this is not a requirement) - hell, once upon a time I ran a whole ISP this way.
Using a NAS, it again depends on the NAS, but yes, a lot of them will do an adequate job - but in most cases they use a similar chip to a Router, so doing it on a router is a better idea.
Bottom line - if you get a mid-high end router and put dd-wrt or similar on it, and you don't need to push more then 50 megabit of traffic and can use an OpenVPN router, it should be more then adequate. If you drop more money on it, you can get even more throughput.
A VPN on a dedicated computer could be a viable solution - but it will consume extra power. Of-course, throw Linux and an extra couple of NICS on it, and its a router. Throw in some disks and its a NAS.