Problem

This problem exists in two variations

My Sim disappears when arriving at the vacation destination. The UI is loaded without the Sim and therefore it’s empty
I travel with a group of people but when arriving at the vacation destination one or more of the Sims are missing.

Cause

This is not really a bug but more the symptom of one. There’s an error in the game that causes the game to not be able to load the interface properly, resulting in the empty UI. There are several issues in the game that can cause this.
Sims that travel together must know each other. Sometimes that is not the case for Sims in the same household if they have never interacted.

Solution

Problem 1

Load a older backup and try again. Make sure to save before going on vacation so that you don’t have to go to far back if you get the error. It is possible that the error occurs reproducible every time though.
If you can reproduce the error every time you try after loading a backup, try switching active families, save the game, then switch back and go traveling again (Credit to davebignell)
Before going traveling, make sure to empty out any items from your family inventory, especially gifts you have received (Celebrity and otherwise). (Credit to DarkAmaranth1966)

Steps you can try if you don’t have a backup or the error occurs reproducible

The following methods might be able to save your town. I did not try those myself, trying is at your own risk.

by <i>QueenBeewannabee</i> in the official US forum

Make a copy of your savegame folder
Go to town edit mode
Click on any random family in town and select them as your active household.
Your active family should now be playable. If you look on the vacation tab that lets you know how much time is left at your destination that means you are good to go at the next step
Click on phone, computer etc and “go home”
Once you arrive home, your original sim family (the one that dissapeared) should be back in place.
Your random sim family from vacation are also now added as members to the family.
Simply kick out additional sim family and you will now have your original family safe and intact.

So far, all my inventory items have been safe however I have always lost my simoleons.

Source

by <i>zerobandwidth</i> in the official US forum

Make a copy of your savegame folder
In the COPY directory delete the .nhd file(s) corresponding to the area(s) where you’re experiencing problems.
Start Sims 3 and open your COPY save.
Choose one member of your household to travel to the location.
Wait for the travel operation to crash Sims 3, taking you back to the desktop.
Start Sims 3 again, and open your COPY save again.
As a test, send one person to the troubled location again.
EXPECT: At this point, the travel process will succeed, and your one sim will land in the remote location, and your Sims 3 dashboard will be intact. The game has recreated the location’s neighborhood data. The location is now playable again, but…
THE PRICE YOU PAY FOR PROGRESS: Any edits you’ve made to the area are now gone, and (this needs verification) all dungeon areas have been reset. After all, you just deleted the neighborhood. All adventure progress is saved with the Sim, so that won’t be lost.
Bring your test sim back home.
If your test sim makes it home, and there are no other observed errors, SAVE YOUR GAME, preferably with another new name.
EXPECT: The new save directory will have a newly-created .nhd file for the location that you tested.
Rinse and repeat for additional locations if desired.

Source (others have posted similar issues and a big thank you goes to Jaghar7, who research a lot on this issue)

Problem 2

Make sure they know each other before sending them on vacation.

Mods that solves the problem

Twallan has re-written the EA system with Traveler. He hasn’t cured every single cause for this issue but if you do have the issue, you get a scriptError which can be posted at Twallan’s wiki.

Missing Sims and empty UI when arriving in vacation town