I have personal sheets and macros added onto IB's TwsDde.xls. Based on a timer, my macro will run at times during the day and spit out an output to IB's Orders page to place a trade. I'm not sure exactly how IB's subscription and ticker code works in the background, but does running an outside macro (and having Excel be frozen for a few seconds) interfere with their subscription code at all?
Excel is a single-threaded application. When you run a macro it freezes. The subscriptions with IB remain but if you have cells being updated their value will freeze until your macro has finished. so if your trade logic references these cells you could have issues, but the subcription itself won't have any problem. if you need your vba process code to run in a seperate thread it is best to write a excel add-in using COM/add-in and have that run in a seperate thread.