[TnTSCS]

This plugin is already designed how you're asking... if I understand you correctly.

If you have sm_grenadepack2_flashbang 2 and sm_grenadepack2_flashbang_vip 4 then regular players can only buy 2 but VIPs can buy 4

If you have:
sm_grenadepack2_flashbang 4
sm_grenadepack2_flashbang_vip 2

then everyone will be able to buy 4

The purpose of this plugin was to allow you to have a VIP feature with grenades on your server so regular players couldn't buy as many as VIP players. VIP players get a little extra treat with the ability to buy more than the regular player