I tried to use "vec_dist" to make different damage from explosion.
Problem is that it works visa versa, I need to invert distance from "vec_dist"?
How to do that? Or may be there is another way which I don't see?

Just make something like this:
if(vec_dist(my.x,you.x)>100&&vec_dist(my.x,you.x)<200)
{
blabla
}

1000 is the maximum damage radius

Alibaba, thank you, but you didn't get me right
I don't need to detect distance, I need to use it to decrease health.
wdlmaster, thank you, I'll give it a try, only one question:
* what number must be used for "some_factor_to_control_the_strength"?
Maximum damage radius for me is 500.

"some_factor_to_control_the_strength" is the damage, which you get in the center of the explosion with my code!
xxxxxxx

OK, now it's clear for me, thank you.