Wondering how can understand that HARM is on my way with VEGA. I've just started to train on it, but I have great dificulties with target aquisition without IADS... Don't even imagine how to deal with US NAVY...
I think the only way is to ask your Tall King.

P-12/15/18 had no problems observing Shrikes and Standards over Hanoi.
A few hints from my painful experience of no-IADS acquisition by Vega:
1) antenna elevation vs. target altitude - click and hold in elevation setting area, then change to Z screen and move mouse left-right, still holding. You will see target altitude indicator following your moves.
2) don't forget to set target's predicted radial speed - I would guess that anything different than SR-71 would have approx 300 m/s, or close to zero if your plotting chart shows that they are going parallel.
3) then, KRUG scanning should do the job.
4) if not, you have LOTS of time to retry. No nerves as by Neva.

Hpasp, in Vega 1985 scenario I shoot target 3304. Using some tricks I have my missile get close to him, as the missile passes predicted impact point the linear indicator gets resetted (no more missile in flight), no KRO accordingly, I can still see the guy getting lower and lower, but of no avail in AAR. I see that you have programmed their reaction by lowering altitude, but - a kill or not?
Have YOU killed anything but SR-71 and your own MiG-25 in this secnario ever?
Thanks for installation hint. More time to practice!