1) In my opinion, the RDS is a superior sight despite being easier to unlock. The dot is far smaller, and it doesn't block your peripheral vision as much as the Holographic does. I favour it at both long range and close, to be honest with you. There is a difference between these sights in real life, based on light reflection I believe, but on the game... not so much.
2) The FAL is single-shot, but despite that, has amazing damage. With Stopping Power equipped, a shot to the head with a FAL equates to that player hitting the dirt. This is superior to every other Assault Rifle, unless you count a burst as a single shot to the head (in the case of the FAMAS / M16). It acts as a good sniper if you equip it with the ACOG or Thermal, because whereas most snipers have recoil, the FAL's is minimal as it's primarily an Assault Rifle, allowing you to fire off additional shots if required.
3) The Vector and UMP are both very good for room clearing and close-range combat. The UMP has higher damage, whereas the Vector utilises a faster fire-rate, but both will kill you in no time at all if you get close enough. They both also have a lot of recoil, so it's really a question of which gun you prefer to use.