When you say truck, what kind? Is it a pickup or tractor? The type of vehicle and your vehicles ground plane play a big part.
VR4 will adjust the modulation only
You will need to check the carrier or deadkey with a rf or power meter to make sure of "watts" going into the amp, too much and it's gonna get hot and somethings gonna go.
Swr meter is used to check and adjust your antenna and coax to the radio.
You can go to Wilson's or firesticks website for more info on antennas and swr.
Then before you go any further remember your antenna system, not just the antenna, but the coax, mounting, etc are what make the system. If you have a $500 radio hooked up with a $5 antenna system it's gonna sound like a $5 radio.
Get your antenna and swr right before you add an amp.
When you are born you get a ticket to the freak show, when your born in America you get a front row seat- George Carlin