I don't think he have as much advantage as you think he does.
The only thing going for him is his 710 range at max W rank. While this is nice, he is below average range without it (Kog have 500, while Vayne have 550 and Jinx 525).
His damage output late game isn't higher than Vayne's at all (not sure about Jinx). The magic damage from Kog'Maw can be easily outdamaged by Vayne's true damage. For example when enemy have 2k health and 70 MR - Vayne will deal more damage than Kog (I do count both per 3 strikes). I'd say that Kog will outdamage Vayne only in very rare cases (against level 18 squishies for example, as they rarely buy MR and their health is anywhere between 1700 and 2000).
Also, Kog have lower dps by himself as well, with 100 AD and 45% bonus AS from levels at 18. Even if we add Q passive (30% attack speed) to it, the dps will look as follows:
Vayne: 120,5 physical damage/s
Kog'Maw: 116,5 physical damage/s
And mind you that Vayne have mobility and even CC, which Kog desperately lacks.
Anyway, my point is - Kog works if you build a team around him, but even then he's not much better than Vayne - if at all.