"Deal damage to Laser Designated enemy vehicles using lock-on weapons"

Unfortunately this is just one of those awful challenges that you have to accept will take a while to get, unless you get lucky. Also make sure you aren't tracking it, because tracking is apparently one of the ways it bugs and doesn't progress.

What annoys me is the first game I tried to get it, with my stinger I did have someone putting painting locks on, and hit vehicles enough to complete it.

But it bugged out.

So I have to do it again and so far I've had one person paint a vehicle once that let me hit it, and a few times where I someone painted but I got killed during lock-on.

It's annoying for sure.

I had a CQ game on Firestorm where I managed to get a few hits with the stinger to get it up by about 150 points. So I'm at around 350/1000 at the moment.

Ask someone in your squad or team?
It sucks but I get why they put these things in: to encourage team play

In addition if you laser designate-paint a target yourself, in the anti-infantry tank, using the gunner secondary and then use the stinger to hit it doesn't count.

Its really badly designed.

I almost only play engi since bf3 and being locked out of cool gadgets because my teammates dont want to soflam is bs. 

I managed to get it tonight. Played a dozen or so conquest or escalation.

Use your own laser as sniper class, worked out for me

That doesn't make any sense. How do you hit something with a launcher as a sniper? Snipers can't use launchers.

its the same for recon i need to laser designate target tha sustain damage ive c4 and then lasered the target and nothing

So the point how to do that challenge is 
Recon should use laser designator with RIGHT CLICK(Binocular)
DO NOT PLACE IT on the ground
recon marks the target
engineer shooting with stinger(even on tanks etc)
ggs
P.S
u not gonna get any points if u gonna shoot with stinger on marked target if the target was marked by recon who placed SOFLAM on the ground
so grab a friend and tell him to use binocular variant of soflam

But how you gonna shot with stinger if you r deploying the laser with sniper class?

get a friend playing recon or ask some recon in the game to do that 
this challenge is not for solo completing as engineer