In real life A-A engagements are a rarity for attack helicopters. Thats why most of them have barely a few choices for A-A weapons.
The Jet VS Helo fights is even rarer. Almost all A-A kills of modern fighters are aircraft and not helos. Only a few kills have been reported against helos (one is a kill of an Iraqi helo which had just taken off... the kill occured with A-G ordnance). Why is that?
The jets has advantages where a helo disadvantages and the reverse. For example, the jet is faster than helo but the last is far more manouverable. Putting yourself (jet pilot) in risk for shooting down a helo is plain stupid! The jet can overshoot quite easily and this would give the more manouverable helo a big advantage to hit the jet. In case a jet pilot attacks a helo (the opposite can hardly happen!), the helo's pilot should avoid it rather than try to engage and get into firing position. In case the jet manages to acquire a lock and launches a missile the helo's chances to evade it are quite high. More or less to shoot down a helo (as someone posted before) isn't something to be proud of... its more like a sign of stupidity for many reasons.
In helo vs helo situations I would prefer the gun over the Vikhr. Put the gun to fire straight forward and select HE rounds. Try to attack the enemy helo from the sides or above. Hopefully the helos most sensitive parts(rotors) will take some damage and this will bring it down. The option of shooting down a helo with your Vikhr missiles is, in my opinion, difficult and expensive and so i avoid to even try it.