frame relay switches forward frames based on one label called dlci. they are not aware about any IP addresses (neither source nor destination)
for example if a packet comes to MPLS switch on port1 with dlci 16 then it must exit the switch via port 3
the dlci mapping is manually configured from the router in customer site
for vpn...it depends what do you mean by VPN: internet VPN or MPLS VPN?
internet VPN means you need to have an internet connection in every site and a device that supports IPSEC protocols to encrypt traffic and send it across the Internet Public net.
MPLS VPN: you need to ask for an MPLS connection from your provider. MPLS switches use also so labels to forward traffic but take decisions based on labels and IP destinations. they are aware about IP addresses. this gives them the advantage to be able to insure QoS (bandwidth, delay) across the path and to extend the QoS in the LAN.
if you have applications that need QoS insurance (like IP telephony...) than you need to connect your sites using MPLS. you need to ask for an MPLS connection in each site from your Telecom Service Provider.
if you already have 3 independant sites, each one having one Internet connection and one device supporting IPSec (router, firewall,...) and you don't care about QoS than you can make Internet VPN. but be carefull bandwidth will not be garanteed. Internet network is a best effort service.
if you don't have Internet connections in each site and you don't care about QoS, then you can go for FR if it is cheaper.
MPLS is the technology that should replace FR in the future, so if they have almost same price go for MPLS connections
ohassairi explain very well what FR, VPN and MPLS VPN are. I just want to add that you have to think to FR as a Layer 2 technology and VPN (I just guest from your simple phrase that you refer to generic VPN term like PPTP, IPsec and so on...) as a Layer 3 technology.
If this is not what you are looking for, let us know what do you understand by VPN.
I will take a slightly different approach to explain the difference than Calin or ohassairi.
The most important difference is that Frame Relay is a layer 2 technology that runs on a physical medium of a serial interface. VPN is a virtual service and is not dependent on the physical media over which it runs (and it can run over many different media).
Another difference is that VPN generally provides protection (or privacy) for all the data that is sent through it. This is frequently done by encrypting the data going through the VPN. Frame Relay does not do anything special to encrypt or provide special protection for the data sent over the Frame Relay.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...