Vanguard vendors have equivalents to crucible gear, you don't need crucible gear, so not getting marks/rep for them isn't ruining your pve experience. And you can still join factions for rank rewards. Rank rewards are honestly better than the vendors' inventories, anyway. While each rank gift isn't guaranteed to contain new gear, the gear you will eventually get often have better perks/stats. And faction gifts can contain loot they don't even sell. Buying from vendors is rather pointless unless you want a so-so legendary immediately. The only downside is being unable to rank up quicker by exchanging mats, which still isn't a big deal - do the nightfall for the weekly bonus xp modifier, then do all your regular pve stuff (dailies, weeklies, patrols, bounties, strikes, publics), and you can hit a new rank each day.