From what I remember from my MCC on A320 ten years ago, which was my only Airbus experience, VNAV PTH is equivalent to the managed modes in the Airbus. It does whatever is entered in the FMC as well slowing down for configuration (very conservatively). It doesn't only work on an ISA day, if you take the time to fill out the DES page with the forecast winds and temperatures.
If you leave the window closed, it'll indeed slow down to the low speed protection, expecting the crew to select flaps on schedule. Most people I know are not huge fans of using VNAV down low, but it's very useful for STARs with lots of altitude and speed limits. Once on the approach, I usually use the basic modes or open the speed window.