I have started a fully customized neighbourhood, using only my own terrains and empty templates, which means that all sims and lots are made by me. However, I would like to make an exception with the secret lots of vacation hoods: I have checked some pages about them, and it seems that there are so many things to take into account when building custom ones that their native NPCs may not appear if I get some detail wrong. Perhaps it may be simpler if I could somehow extract the secret lot from the vacation hood of a test neighbourhood, and move it to the one I'm playing. Is there a way to do that? Would that way have some risk?

It's fairly easy to do as a matter of fact. We discussed it on a thread a while back and I made a lot for the eastern destination with instructions on how to install it here: http://www.modthesims.info/download.php?t=561304

I keep meaning to make secret lots for the other destinations too but I'll see if I can find the thread where we talked about it. It's really not hard - as long as you have the right items on lot, you're generally good to go.

Edit: I found the thread where we were discussing it: http://www.modthesims.info/showthread.php?t=537766
You should find all the information there that you need to build each type. If you run into trouble, I can always try to help (though it's been a while since I built those).

It's been a couple of weeks, but I finally found how to do what I wanted to do, so I will share, just in case it helps someone else.

First, create a new neighbourhood. No sims, no lots. Then, add the three default vacation destinations. Enter each one, and remove all the buildings in it. The secret vacation lots are insible, and will remain. Leave the game.
Now, go to Sims 2/Neighborhoods/N(number of new neighborhood)/Lots. I have all eps and there are still some other lots in there (I suspect that the hobbie lots), but the secret vacation lots are the ones with the higher numbers. Copy the three files, and paste them at Sims 2/LotCatalog. Change their filenames to fit the ones in that folder, and use the numbers that would follow the last one.
Start the game, and load the main neighborhood. The lots are available in the catalog, you can select them and place them. Note that they will become invisible once placed, and that no other lots should be placed above. Remember as well to use the default names. Done? No, not yet. Leave the game.
Now, start SimPe. Go to neighborhood browser, select the neighborhood, and then display the options at the right, to load the sub-neighborhoods. Go to "Lot description" and select the secret lot. "Lot type" will appear as "Community", change that to "VacationHidden". Commit and save.
And that's it.