You could always go Oldschool with the
Anti-Tank Dagger- Is this an AntiPersonel grenade that is armor piercing?
- When you say it is from a Launcher only, is that a small under gun launcher like 40mm m203 or a large defice like the RPG7?
- What is the standard frag grenade damage in your system?
Assuming it is an antipersonel future grenade, and RAW bodyarmors dont have Hardness/ER, just use 50% of the damage as Energy type 5'r RefDC20 and the other half as a seperate Slashing type 15'r RefDC15. The energy damage is not affected by nonHard/ER armors, such as the classic Armor As RD option rule.
But you dont need any slashing, just do it like a energy grenade to leave most armors behind.
One big problem you will encounter is that Armor Piercing is done by concentrating the power on one spot, but grenades are all about spreading out the power. In a modern sense, one can make an ArmorPiercing grenade, but there would be fewer Spikey-er bits with more penetration: A standard 4d6 grenade would get Ignore5 Hard/Armor but a 5 lower RefDC because there are fewer spikes in the air.