Port Vathy is definitely the safest anchorage in Sifnos, if gusty due to the meltemi (the Etesians) (the Etesians). It is a pleasant setting, if somewhat remote from the rest of the island, with a nice sandy beach.
Anchor off anywhere in 3-4 m, or do an anchor moor to the small mole at the North of the cove, where there is space for three yachts in depths of 2.5-3.0 meters (but limited mooring rings). The bottom is sand and provides good holding. A choice of good taverns ashore and a mini-market.
Reaching Fikiada is a small but also pleasant adventure! The joy of the sun and the sea on the sandy beach will be your reward! An environment of extreme beauty will accompany your way to Fikiada. Don't forget that you will need some extra time for your return because of the slightly steep ground.