So did you plan to create more vector icons?
The current plan is to have a seamless combination of bitmap and vector icons. For documents and devices, which are commonly using icons provided by the OS, vector-based graphics would be provided (via Ambient's default icons).
For applications, however, I would like to switch to 256 x 256 PNG icons, which is the maximum display size for icons of any type in Ambient right now (and will likely remain so for a while). The difference in terms of disk space is neglible as, unlike with documents, you tend to have only one single icon file per application so it does not matter too much if the icon file requires 20kb instead of 5kb of disk size.
Why keep bitmap graphics around? There are numerous reasons. If you aim for a certain level of detail, then vector icons can certainly be more taxing. On average, application icons tend to be more detailed so bitmap graphics are a better fit (unless you opt to heavily decrease the amount of detail). Plus, there is currently no suitable vector graphics application available for MorphOS that would allow third-party application developers to design adequate vector icons. Even if there was, I would argue that designing vector-based icons that do not completely deviate from the existing look of MorphOS requires additional effort compared to bitmap graphics design.
The major stumbling block to this plan is that Ambient's bitmap scaling routines need an update to make downscaled icons look a bit sharper so they fit better to vector icons. That said, as you found out, not everybody might even notice the difference as things are now.