Vipers are a special case. We have the Block-72's engine and airframe infrastructure already, and the Block-72 costs the least of all of the options.
Yes, a $6 billion US program to buy 36 Block-72s and upgrade our existing F-16s to F-16V standards would take money away from Proj. Azm, but the net-benefit is adding 100+ AESA radar-equipped jets -- that's an OK trade-off.
But spending $3-4 b on 30-40 Su-35s or 18-24 Typhoon T3s isn't a good trade-off for delaying Project Azm. If those are the only options, then the PAF will eat grass and work on Project Azm.
As for the Chinese, if low cost enough, we can look at leasing their Flanker derivatives if the option is available. But we won't keep them when we have our own twin-engine jet in the works.
Su-35 &Typhoon - Are out. PAF has closed this chapter
J-15 - Not sure how the funding will go, for now. But I believe it will be heavily subsidized.