Jim's 1st rule of photography:
Lenses rule, a camera is just a light-tight box.
Translation:
The lens really makes the picture (may be some argument talking digital SLRs, but that's another subject). Now the question is, what do you intend to do with the pictures you take? An inexpensive lens may well be perfect for touristy "I was there" type pics and then be totally unacceptable for printing 8x10s.
Examine your intended outcome and then determine if a particular lens will fit that need. If you will use a 600mm lens 5 times a year it doesn't make much sense to spend big bucks on a dust collector. OTOH, if you plan to shoot action shots weekly you would be better served buying a good (read that as expensive) lens.
And look beyond your immediate needs as well. Buying an inexpensive lens and then replacing it is more costly than saving for the better lens from the start. It is a two edged sword though. While you're saving for the better lens you could be taking pictures with the inexpensive one.
Now to directly address your question (finally, eh?)- Vivitar has been around for some time. Their lenses run the gamut from so-so to pretty damn good, so it's a crap shoot on what your getting. The Series One lenses were pretty good glass in their day. The one you're looking at is a mirror lens I think, so you need to factor that in as well.
Phoenix is a relatively new maker from Korea (I believe) that also brands for Samyang (and maybe others). Being so new I haven't seen much about them.
hth [/end rambling confusing post] :shock:
Jim