Is there any reason to use those weapons?
The Flak has a such short range that is almost suicidal to use it. The Vortex I am not even sure of what it does?

Did you manage to use this weapons? How so?

There's an upgrade for the Flak ("VK") that gives it extra range and which I recommend. The Flak can be a good room clearing tool in medium to close quarters, but it's still a bit of a specialty weapon. I used it mostly in the Secret Levels to have a primary weapon I could just wildly spray without aiming.

As for the Vortex, I haven't been able to put it to good use, so I'd also appreciate some tips on its operation. I thought its vacuum effect would cancel the final enemy teleports, but it doesn't seem to do that.