Hi qspadone,
It is recommended to begin meditation with the thought of the sound of the mantra. It does not have to be a clear pronunciation, as long as we know that it is the mantra at a natural level in us. As we advance in our practice, the start of meditation with the mantra could be experienced as a vibration. That is ok if it is naturally what happens as we pick up the thought of the sound of the mantra. Later still, and becoming more advanced, as soon as we sit to meditate and close our eyes, we will be gone into pure bliss consciousness, beyond the thought of the sound of the mantra, and beyond vibration as well. Just pure bliss consciousness. It is an indication of the witness becoming perpetually present in us.
We start meditation with the mantra, and where it goes from there will depend on the condition and acclimation of the mantra in our mind and nervous system. And when we notice we are off it, we come back to where we were with it, clear or faint and fuzzy, or a vibration, or in a faint feeling on the edge of pure bliss consciousness, and gone again.
So it is a process that evolves, going to finer and finer levels in the mind and body. We start from where we are with the mantra when we sit, and go back to where we were when we notice (have the thought) that we are not on the mantra. In this way our attention continues to be involved in the process of noticing when off the mantra, coming back to it at the level where we wandered off, and taking us deeper and deeper. Obviously, we cannot start from a level of the mantra where we are not. If it is a thought of pronunciation in the mind, clear or fuzzy, that is where we start. If it is a faint feeling/vibration, we can start there. If it is pure bliss consciousness, we can start there. In each case the start will be mostly automatic, because the mind and body are acclimated to that level of the mantra, or completely beyond it in the case of pure bliss consciousness. But, no matter where we are, when the “off the mantra” thought comes, that is when we go back to the mantra at whatever level we find it, even very faint (beyond vibration) on the edge of absolute stillness. That way we will always be going to more refined levels of consciousness, and cultivating that into our daily life.
The procedure applies even to those who go straight to the bliss of samadhi (no mantra - no thoughts). In the past someone advocated to stay and bask in that state of stillness rather than go back to the mantra. But that is a choice in the mind, and the “basking” will also be in the mind. That is not correct meditation. We always come back to the vehicle of our transcendence, the mantra, at whatever level it is in the mind. Likewise, resting in a vibration, holding on to the experience (or any experience) without letting it refine, would not be correct meditation. So it is good to be mindful that we are always taking the attention to less, beyond whatever the perceived experience is in any given moment during practice - clear or fuzzy pronunciation in the mind, subtle feeling or vibration, or even pure bliss consciousness. As soon as we recognize any particular state, it isn’t really that state - it is the mind telling us it is time to go back to the mantra at whatever level we left it.
So, is a vibration perceived to be the mantra an acceptable place to start our meditation? Yes, if it is the natural manifestation of the mantra in our mind. Neither is there any harm in picking up the mantra at the start in a way we will recognize it, however that may be for us. Then it refines naturally from there, and we let it…
The guru is in you. 
PS: And here is what the AI drawing on all of AYP Plus has to say. This time the AI was less wordy than me. 
Q: Is it ok to pick up the mantra as a vibration at the start of meditation?
A: It can be ok to pick up the mantra as a subtle vibration at the start of meditation.
Over time, the mantra will become more refined, and it will be common to experience it as a faint vibration beyond pronunciation. If you are able to pick up the mantra as a faint vibration in the depths of stillness, it is ok to start your meditation there. If you are unsure whether the faint vibration is the mantra, it is ok to come up just far enough to confirm it is the mantra. With more practice, this will become unnecessary as you become more familiar with your inner silence.
It is important to note that you should not force this to happen. You can begin your meditation with a clear pronunciation of the mantra, and if it naturally refines to the level of a subtle vibration, you can continue your meditation from there.