That's not really true for anything vehicle related though. While the difference between 2 infantry players is mostly certs and as such not the amount of money you spent, besides the fact you can get certs twice as fast if you have 6month membership+50% boost, or at least 75% as far with just 1month membership and that underbarrel grenade launchers can make a difference every now and then and require specific weapons which cost 1000certs.
However when it comes to vehciles, there's major differences depending on wether or not you bought different weapons, which all can be bought with money obviously. An ESF with Rocket pods and without are 2 different beasts, so is one with A2A missiles.
Lightning base gun is complete shit compared to HE or HEAT. HE on MBTs can also be quite useful but to be honest they start with HEAT so they're pretty decent, however the secondary cannon makes a huge difference if you're Vanu and buy the Saron(which is like a railgun, does high vehicle dmg and instantly kill infantry assuming you get direct hits).
Liberators come equipped with complete shit weapons, and while the driver tank gun thing only costs 100certs, the Zelpher and Dalton both cost 1000certs and are way way way better than the basic 2nd seat gun, so much that it actually makes the Lib worth using while without them it's definitely not unless you need personnal transport and your ESF is on cd.
Don't know much about Gal or Sunderer but the Bulldog was a lot better than the base canons on the Sunderer, not sure if it's still the case since it was recently nerfed. Flash is obviously better with a weapon since it comes by default without one, but the 2 machine guns are pretty cheap only the grenade launcher is expensive so not a big deal.
Still overall, there's a huge difference in most cases between the vehicle of someone who paid money OR farmed 1000certs(almost everything good I listed is 1000certs btw) and someone with the stock vehicle. Only exception I find are NC and TR MBTs which perform adequately even without any upgrade since their secondary guns are pretty shit afaik and the basic HEAT is good enough.
Highly recommend the Vehicles pack, it's 20$ instead of 49$ for the stuff separately, and it had almost everything good. The pack might change over time though I'm not sure but the current one since I checked earlier is Rocket Pods for ESF, Zelpher for Lib, HE for Lightning, HE for MBT, Basilisk for Flash, some shit for Gal and/or sunderer.