I have run into the same problem, but I'm coming from Dimension Strings.
Even the normal attack long articulations need some time to get going which makes it really hard for me to use them in melodies with short rests, because the first note after the rest will always have this slow attack. There seems to be no way to start a phrase instantly without using the marcato articulation, which is really not what you want to do.
I've attached a picture which should highlight the issue. The upper waveform is SSP long regular vibrato normal attack, the lower waveform is Dimension Strings sustain vibrato. Both are playing the exact same note at max velocity/CC1. You can clearly see that SSP needs around half a second to get to its peak loudness. DS just comes full force right off the bat.
It only affects the first note because all the following notes in a legato articulation will all have the correct loudness right from the beginning (playing melodies with the non-legato sustain articulations is entirely impossible though). However, this also introduces audible artifacts: If you play the second note while the first is still "starting up" you will notice a sharp change in loudness between the first and the second note.
I really, really love everything else about SSP, but this slow attack on a first long note is driving me nuts. I've been mailing back and forth with VSL support but so far I've not been able to find a viable workaround. What we would need is some sort of "fast" attack articulation. Maybe something could be imported from Synchron Strings I?