Assignments can be optional challenges marines could take for the cost of vendor points, that would refund the cost plus 50/75/100 percent bonus. They would be split up into easy normal and hard, and any squad marine could take them on for more vendies (maybe just start with boots to test.) The challenges would focus on things other than getting kills, the utility and support stuff that marines do should be more rewarded than they currently are; extinguishing teammates, staying near the SL for x seconds, stunning a xenomorph that ends up dead in the next 3 seconds, performing CPR on a defibbable marine, etc.
Killing xenomorphs is often finicky, and almost always the result of a collaberative effort, so instead of promoting a last hit mentality, the assignments would focus on behaviors that help the team rather than the individual's ego, with the harder challenges being for the mucho robustos, and easier challenges being available to stack up for a couple of quick boosts to resupply special ammo and what not.
The individual assignments would be pieces of paper that can fit into document pouches, they would display current progress to completion, and once completed, you'd need to have the ID that bought them on you when you turn them into the prep vendors for the vendor points, so you need to make sure not to lose the paper or your ID.
Completing enough easy assignments might give you a restricted weapon or kit, completing enough medium assignments could give you a random skill manual that stacks, and completing enough hard assignments could give you a spec kit. These values and rewards can obviously be changed around as needed, I'm just spitballing here.
I really like this idea and I think it gives marines some progression to look forward to without it being obnoxious, and would reward the marine for helping their team and surviving to make it to the surface. The command APC might have a machine to cash in these asssignments to prevent people from just abandoning the battlefield every few minutes.