BlueLight said:
I would say yes, you can do it that way. Any kind of mantra can be vibrated mentally. This has to do with vibrating it on the astral from what I know, with your ethereal-/energy-body.
Bear in mind however that you will severely lose out on the power that physical vibration brings. In order to atleast make up for this a little bit, you should try to work on all the other components that go into vibrating the runes and doing workings in general.
Practice your focus and visualization. While you vibrate them mentally, try to focus fully, 100%, on the rune you are vibrating. In addition to this, focus on that rune's symbol in your mind. If you can't visualize well enough yet to properly keep the symbol in your head, then start practicing it until you are able to. I remember that some other members visualized the symbol lighting up brightly with white-gold energy.
When you are unable to utilize physical vibrations, the other parts of raising energy become all the more crucial. I would highly advise that you put time into deeply mastering the act of breathing in energy from different sources (planets, sigils, etc). This can then also be added into your workings. While vibrating the rune like I said above, with full focus, and visualizing it's symbol, you can try drawing in energy from that symbol with each inhale while you vibrate (similarly to doing energy-breathing on a sigil). This should give it a bit of an additional "boost".
Of course as with all things, first give these things a try (preferrably as soon as you can, but not during the VoC) and see how they feel for you. Then you can adjust it to your personal preference and go into uni well prepared.
Best of luck.