Do baluns help improve the performance of VHF/UHF transceivers?
They don’t necessarily improve the
transceiver, but they
can improve the performance of a station when the antenna
isn’t properly balanced.
Are there any VHF/UHF antennas that are NOT 50 ohms impedance ?
Lots of them... depending on what you mean by your question.
Many yagi designs have impedances between about 8 and 30
ohms, for example. But some sort of matching is used between
the antenna and the feedline to give a 50 (or 75) ohm match.
Whether you think of that matching as part of the antenna or
the feed system would change the answer to your question,
but it is a distinction without a difference.
Because feedline losses tend to be more of an issue at VHF
(and higher frequencies) than at HF, ham antennas are
generally matched at the antenna feedpoint, rather than
using an antenna tuner in the shack. Some of the older
designs used twinlead or open wire line, and I remember
several tuner designs for 2m and 6m. But low loss coax
is more commonly available, and much more convenient,
so VHF tuners in the shack are rare these days.