As Jeff refers to, they key thing about the legendary soft focus filters (ie the ones that go on the lens) and lenses is that they produce a sharp image along with the softer image. They aren't all-over soft. Hence the blending of sharp and unsharp layers in PS. Try adding some glow as well as blur.
One of the easiest and cheapest soft focus filters for camera use is the well-known stocking over the lens. Tights, stockings, panty-hose whatever you want to call them. Different colours have different effects - black is the most common. You can get them cast in resin, should you so desire.
The Zeiss Softar can be imitated by dropping small globs of clear glue onto a sacrificial Haze or UV filter - so that there are large portions of the filter that remain unmodified, with little glue lenslets here and there, in an even manner.
The Vaseline trick tended to be more for getting a clear central spot than an overall soft focus effect.