[TheDS1337]

Looks like you forgot to get the pWarpper pointer. that is the first thing I noticed, try this:

PHP Code:

static cell_t Native_IsPlayer(IPluginContext* pContext, const cell_t* params)
{
    CBaseEntity* pEntity = GetCBaseEntity(params[1], false);
    if (!pEntity)
    {
        return pContext->ThrowNativeError("Entity index %d is invalid", params[1]);
    }

    static ICallWrapper* pWrapper = nullptr;
    if (!pWrapper)
    {
        REGISTER_NATIVE_OFFSET("CBaseEntity::IsPlayer",
            PassInfo retInfo;
            retInfo.flags = PASSFLAG_BYVAL; \
            retInfo.size = sizeof(bool); \
            retInfo.type = PassType_Basic; \
            pWrapper  = g_pBinTools->CreateVCall(offset, 0, 0, &retInfo, nullptr, 0));
    }

    if( !pWrapper )
    {
         return pContext->ThrowNativeError("Failed to get ICallWarpper pointer");
    }

    //ArgBuffer<CBaseEntity*> vstk(pEntity);

    unsigned char vstk[sizeof(CBaseEntity*)];
    unsigned char* vptr = vstk;

    *(CBaseEntity**)vptr = pEntity;

    bool bResult;
    pWrapper->Execute(vstk, &bResult);
    return (bResult == true) ? 1 : 0;
} 


EDIT: Ah, one more thing. don't forget to free it, I think using a List or a Vector to store the pointer and free it on extension unload is a good idea.