Ehhh.. a FIX engine coded in VBA? What's the point? I guess it depends on your strategy requirements, if you're only firing once a day then sure this thing would probably work. I presume you're not trading proper high-frequency, but even medium frequency I have no idea what this would even be on the table for a solution to consider. That being said, if you're seriously tied to excel, I've seen one reasonable excel-ish implementation. Basically what they did was coded up a basic OMS (took care of agging, order sizing, etc), that would dump out a trade file in csv format, then they would load it into EMSX for trading which was connected via FIX. Though they were only doing daily rebalancing.