I've had very bad experience with splitters that kept dying or locking onto the wrong path permanently, on several boats. We had the VHF antenna on the mast top and the AIS antenna on the stern arch, which gave us no problems apart from a faulty connection once. Also if the mast had ever fallen over the side, we would've had a backup antenna ready to plug over.
A VHF antenna is normally tuned to channel 16, which is at 156.8 MHz, while AIS uses 162 MHz. A VHF antenna will work, but one tuned specifically for AIS will work better. You can do this yourself though, by chopping a few carefully calculated millimeters off a standard VHF antenna. I did so and found a notable improvement in AIS range. It's not critically important though. The splitter is likely to cause trouble, although I haven't heard one of the emtrak integrated ones fail (yet) - they're fairly new though! The Simrad ones were notorious for it.