Who is better to get kogmaw or vayne?

I would Pref Kog

Have you tried both of them? What are you looking for in a Marks(wo)man? If you want my opinion, I think you should go for because I think he is more fun (AP Kog'Maw is a reason) but you must be warned, he needs a team that can help protect him.

I haven't tried the new Kog'Maw, but Vayne has a higher skill cap. I would personally suggest Kog, who in my experience is easier to play. Also, living bioweapon machine gun artillery. How does that not sound awesome?

I personally find vayne boring, 3 auto attacks, for percentage damage, not really fun, so I'm biased.

for the win

Right now vayne is better (though this could be because people haven't figured out how to play kog'maw since the recent changes), but kog'maw should be getting buffed soon.

They have very different playstyles; i'm a bit surprised you are deciding between them.

Kog'maw is a behind-the-front-lines, very immobile, artillery piece that requires his team to coordinate and protect him in order to rain down damage on the enemies from afar.

Vayne is the duelist extraodinaire among marksmen. Highly elusive and mobile but shortranged, she's just about as far from kog'maw as you can get.

Kog'Maw is shit right now. Get Vayne, or wait untill he gets buffed (which they probably won't ever do).