What version of android are you running? 4.2 has a lot of issues, including bluetooth. I have used 4.0 and 4.1 and been ok, I have not tried to run vemstune on 4.2 because I uninstalled it almost immediately as its really beta, but the support explicitly called out bluetooth as being buggy.
I would try to connect to your tablet to a laptop with a known good bluetooth chip. Should be easy. You can set up a virtual bluetooth com port in windows with one of those serial port communication apps (like teraterm or similar). Then load up a bluetooth terminal on your android phone. You should be able to connect them and send messages back and forth. This will confirm that the android tablet bluetooth is good.
Then, you can use a USB to Serial adapter to connect the serial to bluetooth adapter that you have to your computer. You should be able to run the same test as above, but using the serial to bluetooth adapter. This should work if the tx/rx lines are the wrong way around (not sure if it will work if they are the right way, I don't recall testing this with mine...) This should confirm that the android tablet can connect to your bluetooth adapter. If it can, then you know you have an issue with your VEMS wiring or settings on the adapter or maybe power to the adapter.
After that, ensure you are swapping the right pins on the com connector. Some places like to list pin numbers looking at the connector from the solder joints, others from the plug side... Make sure you are looking at it the right way.
If it still doesn't work, I would start reading up on com ports or try a different adapter.
Jim