I think there are quite a few mistakes in your code :
(1) InventoryInfo class does not have any member func named WriteToFile !
Code:
int main()
{
        vector<InventoryInfo> iInfo;  //Creates an object...vector of strings...
        Fill(iInfo);
        Show(iInfo);
	Remove(iInfo);
	Show(iInfo);
	Find(iInfo);
	iInfo.WriteToFile("Success");
}
So, use it like : WriteToFile(iInfo);

(2) Vector does not have a member named c_str() !! Strings have it.
Code:
void WriteToFile(vector<InventoryInfo> iInfo)
{
	fstream OutFile(iInfo.c_str(), ios::out);
	vector<InventoryInfo>::iterator AccessInventory; //Sets up an iterator...
	for( AccessInventory = iInfo.begin() ; AccessInventory != iInfo.end() ; ++AccessInventory)
	{
		OutFile<<AccessInventory->GetID()<<endl;
		OutFile<<AccessInventory->GetDesc()<<endl;
		OutFile<<AccessInventory->GetCost()<<endl;
	}
	OutFile.close();
}
I really don't understand what you wanted to do here !
Why did you need to pass "Success" into this func ?? Did you intend it to write to a file named "Success" ??
In that case, change the line to fstream OutFile("Success", ios::out); and don't pass "Success" as an argument, rather pass iInfo as the argument.