A short update (for everyone who's interested - which is much appreciated).
A couple of days after my last post I entered a rather sucky period (1-2 weeks). The perception of vibrations seemed to fade. I lost access to what I think may be the first samatha jhana. I started to get quite a bit obsessed with where I am on the path, even developed a sense of desperation, like "I won't make it, I just don't have the abilities, etc. ..." This was topped with the return of physical pain during meditation: a cramped left shoulder which produced a tight and annoying pain which at times radiated out in the skull (on which it presented itself sometimes in only a square centimeter of the surface - never had that before). Not as strong as I've experienced before, mostly present during walking meditation, during the sittings more in the background.
After a while I realized that I was pretty attached to what I saw as signs of progress (perceiving vibrations, seemingly access to the first samatha jhana) and how this grasping tightened me up. I was able to loosen the grip (mainly through focusing on the emotions around it) and dropped out of it (without actively trying to do that). The time afterwards was great! Even during my vipassana sits I was under the impression that I was entering the first samatha jhana (without pursuing it). And everything went "silky" which was a rather nice experience: I had a few days in which all of my sensory perceptions felt extremely smooth, like playing with a piece of silk. Also, my tongue felt at times as if it was under an electrical current - like licking on a battery.

In hindsight the whole episode looks like I stumbled deep into the 3Cs (or lower) and after a while made my way back to near A&P territory. I still don't believe that I went through A&P (at the beginning of my samatha sits I usually have a jerky breathing pattern, and I often have what Daniel describes in MCTB as ratchet-like sensations when I'm walking throughout the day (it took me quite some time to link this sensation to the term "ratchet-like")).
Another thing I'd like to describe: Over the last weeks, mainly during walking (meditation, but also at other times), I've been experiencing an increasing amount of what I call "floating." I sometimes felt like I'm on a ship in a very stormy sea. I'm not actually swaying / stumbling / loosing my balance - it's just a mental impression, as if the observer stays a bit behind the actual movement. At the beginning, it felt like I was passing through some narrow and unexpected imbalances of the gravity field which pushed me forward and/or down or held me back. It also had a bit of an annoying component, even made me feel like I'm on the verge of nausea (like being drunk). At its peak (regarding the impact) I once walked down a staircase and suddenly had the impression that the staircase was pushing itself upward with full power onto my feet and I frantically grabbed the handrail.

Over time it became smoother, much broader and started to be rather like a gentle floating / swaying, a bit like moonwalking. Sometimes there's a similar experience at the beginning of a sitting (samatha and vipassana): it's a bit like I'm in a slow and wide whirl, also very gentle and without any nausea generating effect. Funny stuff, but I'm under the impression that this are just side effects, not really producing any insight (aside from noting the 3Cs of this experience too).
Right now I feel like I'm gaining stability in my samatha practice. It still takes quite some time to reach the point at which I think that the first jhana is reached (it may as well be just access concentration, but its very pleasurable), but the ability to grab the meditation object (mantra), immerse myself in it and stay with it longer and longer without interruption is increasing. My vipassana sits are much quieter as at the time of my last post (at that time some of them where pretty wild, like everything (internally) shaking all over the place). Still, at least during the last days, I'm reaching sometimes a point at which I'm under the impression that there's something "big" ahead. Usually accompanied by a speeding up of the frequency of the internal sound vibrations and an increased flickering of the visual field. But usually nothing happens.
For the fourth month in a row now I'm still perceiving vibrations (at a pretty steady frequency) almost all the time in my everyday life. They tend to move in the background of my perception when I'm under strong stress, like last week at work, but seem to be always there.
Unfortunately, I didn't get a place at the retreat I was planing to attend in March. So I'm going to do it by myself (11 days). It's the first time I'm doing a several-day retreat by myself, but I'm confident that it will work.
Thanks for reading. Any comments welcome!