There is more than the typical viruses. Anyone remember cryptolocker that locks your computer courtesy of the "fbi"? There are other things. Online voters can program robots to vote for people crashing the servers. People can even ddos shutdown the servers if they are smart enough. Anything can be exploited from this to even flipping a coin.
Electronic voting machines are open to hacking. Think about how many computer systems and phones get hacked each year. Most of that is done just for fun. Now imagine someone had a real reason to hack a voting machine, there is not likely to be much to stop them at this time.
No, I do not think that electronic voting systems are vulnerable to hacking and are very secure machines. Voting machines have come a long way, and in one time in history may have been able to be hacked into but nowadays it seems as though technology has advanced so that they can't be hacked now.