Vesper Splitter and AIS Targets

The vessel has an XB-8000 transponder, if a Vesper splitter is used, rather than run another antenna, will the AIS targets disappear when transmitting on the  VHF, or is there a lag time to prevent this? 

No, the targets won't disappear right away from either the iPhone app or other devices. The way this works is AIS is received over time. All vessels transmit on period intervals. If a transmission is missed, all the devices will keep the old info until it is subsequently updated. If a long period of time (this varies by device) goes by without receiving from a vessel then that vessel is "timed out" and is often marked with a cross or line through it (that's what a WatchMate 850 or Vision will do) and eventually it will disappear. Of course, as soon as it's received again it will immediately reappear.

When neither the VHF nor the AIS are transmitting then both will share the antenna for receive. This is the normal case and they both receive simultaneously. But of course when one transmits it blocks the reception and transmission from the other. The AIS transmits very briefly so it's not even noticeable on the VHF, but the VHF has transmission priority at all times and if the VHF is transmitting, the AIS is effectively disconnected from the antenna.