Vision score - killing enemy wards and placing control wards will have a much larger impact than simply placing another ward. That said, I'd worry less about vision score than if you are placing the RIGHT wards and positioning properly.
I can't remember names off the top of my head, but there have been a few pro teams over the years who have had maybe 60% vision score compared to other top pro teams and yet they've been just as good or better than these other teams. Why? Vision can also be garnered through inference and other means. Keeping enemies clearing waves under tower grants you vision w/o wards. Fighting the enemy team in their jg and pushing them back to base grants you vision without wards. You see what I'm getting at? That's not to say vision isn't important... it very much is. Vision is what let's you know if you're getting a pick onto an overextended enemy of if he's baiting you into a trap. Vision let's you know the enemy mid is missing and she's roaming bot with the jg to kill you vs she's recalling to base. Vision let's you force the enemy to hard commit to stop you from taking a late game baron to win the game - enabling you to get a decisive teamfight where you have the advantage of surprise.
As a policy, when I'm pushing up the lane with my team, I drop flanking wards at key intersections and bushes in the enemy jg by that lane so I can spot if the enemy is rotating to flank us. It let's us push up safely and also let's us rotate between 2 lanes and catch any enemies off guard if they try to do the same. So it is quality of your vision that matters just as much if not more than quantity. As Jatt and Kobe once said (pretty sure it was them), you can buy a control ward and drop it on your fountain to say... hey I've done my part contributing to vision, but that doesn't really do your team any good.
So hey, props to you looking at how to improve. Keep the first paragraph in mind if all you want is to improve your score, but keep the rest in mind if you truly want to be a better player too. Good luck.