Are vanillin/vanilla powder safe to use in savory dishes?
The vanilla chapter of Ian Hemphill's "Spice and Herb Bible" says,


  Vanilla is also delicious in savory cooking because it is not overtly
  sweet. An innovative Mauritian restaurant in Sydney served a delicious
  vanilla chicken [...] It was aromatic delicate and beautifully
  balanced.


The same book has a recipe for Vanilla Salmon Salad where vanilla seeds are scraped into the dressing.

I did a quick taste test using vanilla extract with various savory things from my fridge. Here are the results:


Cheese: tasted fine, but I wouldn't call it a taste sensation;
Vegeburger: nasty combination; I think the leeks in the burger were a problem;
Avocado: actually quite a nice combination;
Roasted aubergine: not bad;
Tomato: fine;
Savory omelet: not good; onions don't seem to work with vanilla extract;
Baked beans in tomato sauce: very nice.


Note that using vanilla extract and a vanilla bean to flavor a dish are not the same thing. There is an amount of alcohol in the vanilla extract I used (bourbon) and this may completely invalidate the test. Having said that, the first indications are that alliums (onions, leeks, garlic etc) don't go with vanilla. So onion soup may not be a good plan. And since curry generally contains plenty of onion and garlic, I'm skeptical about using it there too.  

Turning to the question of whether vanilla and vanillin become dangerous when heated, the answer is almost certainly no. That doesn't mean that heating vanilla is inconsequential. Harold McGee's "On Food and Cooking" mentions that vanilla beans contain more than 200 volatile compounds and notes,


  Prepared vanilla extracts [...] are best added toward the end of
  cooking; any period of time spent at a high temperature causes aroma
  loss.


On the other hand, heating vanillin at normal cooking temperatures (i.e. normal for soup and curry) is not a problem. Its boiling point is around 285°C.

As another answer mentions, vanilla has a more complex taste to pure vanillin. Most of what you will read comparing the two will be very much in favor of using real vanilla beans in cooking. Instead of repeating this advice, I'm reminded of having read that in a Cook's Illustrated taste test comparing genuine vanilla to synthetic vanillin, the artificial flavoring came out on top. It's probably not be enough to conclude that vanillin tastes better than vanilla but may encourage you to forge ahead if vanillin is all you can afford.

Update: Having read @papin's suggestion that you may need to caramelize the onions, I just did a quick taste test with rostad lök (not sure how it translates from Swedish, the literal translation is toasted onion). It tasted slightly better but still not great. Again, I'm using vanilla extract so my taste test is not necessarily valid.