The RX580 requires a 500w PSU. THe VX is 550w. You should be okay, but I would never recommend that unit to anyone unless on a very strict budget. There is a reason they are quite cheap. They are not the best. If the PSU is younger than 3 yrs, then I'd say you should be okay, but think about changing it within a year. If it's over 3 years old, Id get a new one before putting in the new card. I wouldn't want a shiny new card murdered by a failing PSU.
If you are buying the PSU new, then give us a budget, and we can maybe find something better for you.