While inspecting my cruise control Vac hoses, I noted that my MAF is not plugged in. (The plug is just dangling round beside it) what is the benefit of this? should I plug it back in? Or just leave it

If it is not plugged in then your fuel mixture will be running rich and you will be burning more fuel than you need. You might possibly notice a little surging as you come off the throttle as you approach a traffic intersection. Mind you, if thats been hanging off since you got the car then maybe the previous owner pulled it off for a reason... perhaps it's stuffed - who knows?
Get it checked out as it will be hurting your fuel economy.

I thought it might have been some cheap performance mod or something... I'll just plug it back and see how I go. Keep in mind I am not sure if it was disconnected because of the duel fuel conversion...

Well if its dual fuel then who knows how its setup, maybe thats why they left it off?

But if it still runs on petrol then you definitly want the MAF plugged in, having it removed will not only run the engine pig rich on petrol but will also lock timing adavnce in both petrol and gass modes. The only reason someone would purposly remove it is, as vic said if the MAF is faulty which normally makes the engine go lean which is obviously far more dangerous than running rich. Plug it back in and take it for a hard drive on petrol and listen for any pinging.