[Freesteyelz]

1. Without going into vB's avatar permissions and Usergroup avatar option, I'd create a custom field that is private, only the Admin can see and edit. This custom field is single text box style and named "Custom Avatar". With a conditional I'll set the Usergroup with avatar permissions to override all displayes of avatars, only available for that group. So when the subscriptions ends and the user goes back to a Usergroup without avatar permission the avatar will not display. Though, the text box field will still have the link to the avatar but it won't matter since it is hidden from their profile options in User CP.

The conditional in the postbit or postbit_legacy should be like:

<if condition="is_member_of($bbuserinfo, x) AND $post['fieldy']">
  $post['fieldy']
<else /></if>

Where x is replace it with the Usergroup that has avatar permissions. To add groups just add a comma after every number ID.

Where y is replace it with the custom field ID.

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2. Or if you want to use vB's default avatar options you can try:

In postbit or postbit_legacy find:

   <if condition="$show['avatar']"><td class="alt2"><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$post[avatarurl]" $post[avwidth] $post[avheight] alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></td></if>

And replace it with:

   <if condition="is_member_of($bbuserinfo, x) AND $show['avatar']"><td class="alt2"><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$post[avatarurl]" $post[avwidth] $post[avheight] alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></td></if>

Again, here with x just replace it with the Usergroup ID with avatar permissions.

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One of these options should work.