You're missing the same adapter plate seal I was missing in the drive shaft housing sub assembly. Well, not missing, but chances are the adapter plate has cut through the side of the seal rendering it useless. Manufacturer's part # 87357 which is, of course, discontinued. I found mine on Ebay. You could for the time being get a piece of 1/2" vinyl hose & cut it lengthwise on one side & fit it in place of your worn out seal until you find a new one. You might need to set a piece of 1/4" hose cut the same way inside the 1/2" to make it thick enough to hold up once in place. It will be a temporary fix but should help with the vibration in the mean time.
Don't buy this one
https://www.ebay.com/itm/USED-MERCURY-MARINER-99702A3-ADAPTOR-PLATE-87357-SEAL-96248A1-WATER-TUBE-HOUSING-/281695034399?hash=item4196551c1f:g:6V0AAOSwZd1VWKSc&vxp=mtr
In the 4th picture you can see where the adapter plate has cut through the side of the seal - that's what's happened to yours.
Try some used parts suppliers, https://www.northernoutboard.com/ might have one, I've ordered parts from him in the past & they've come in great condition.
Replacing it will fix the cowl vibration issues you were experiencing