Well I recommend this.
If you are trying to only give subscribers access then you set registered users not able to upload ads period.
If your trying to give registered users pay for every ad and subscribers no payment then set subscribers to no pay.
If you are trying to give subscribers a higher max ads then I suggest registered users 1 and subscribers a higher amount.
In every scenerio if you put a user into multiple users the best scenerio is allowed regarding max ads or payment etc.
Thus it is best when dealing with subscriptions that you set in your vbulletin subscriptions the users primary group be the subscription group and that they have no secondary groups if your going to insist on making a lesser group have the best access scenerio. I hope you follow this here
It is not going to do you no good giving a user who subscribes and throw them into a secondary usergroup for the subscription and there main usergroup already has the best allowed permission as in max ads for usergroup registered is 0 then setting a max ad of 1 for a secondary group just is not going to work here.