This completely varies for each person, it solely depends on your computers specifications. Obviously if you can run 8 Virtual Machines and have decent loading times, I would suggest cow farming however:
Cow farming (8 cows - 5 minutes) wont give you much EXP with just 1 VVIP. I think it would be around 2900, so if you cow farm you will need more than 2-3 VVIPs.
Complete badges, achievements, easier ribbons, and maintain a good kill death ratio, fast daily missions.
You will need atleast 3 VVIPs to hit around 9000 EXP, bare in mind the most EXP I have seen in TDM is 25,000 with an almost full room of 4 VVIPs.
Nade farming will give you more EXP with 1 VVIP because your bonus from the gun as well as others will stack towards your final bonus. However it is boring as fuck and will ruin your kill death ratio.
Greater EXP with 1 VVIP
Massive GP gain
Slower than TDM
You will be kicked for having 1 VVIP
Too many people troll Grenade Farm matches