It isn't being interpreted as an ESC, or at least not *just* as an ESC. I opened up vi in connectbot and tried it. Created a file foo, entered insert mode, typed in bar, alt+a (the "tab" key), got "E29: No previously inserted text." It did, however, exit insert mode. Re-entered insert mode, hit the tab key again, it exited insert mode and redid my last insert (shifted over a char because of the escape), so I ended with babarr.
So, I feel like I'm thinking out loud, but bear with me. In vi, the '.' key redoes the last thing you do, including inserts. enter insert, type bar, press escape, press ., it will output bar into your file. So, it's like it could be entering escape AND '.' . Now, going further off this... by default, the droid replaces 2 spaces with a period (if you're texting and hit space bar twice it will replace the first with a period). See where I'm going with this? It depends on the application, but tab can be recognized as a sequence of spaces (which could be causing vi to think . is being input, hence the text output). Where the escape to exit insert mode is coming from I have no idea.
As to the idea Microsoft patented it, try using a Mac or using Linux. Tab works just fine. It shouldn't be a limitation of the OS, it more likely has something to do with the keyboard (or the keyboards driver).