The Voynich-Ms as a concatenation of abbreviations

[tavie]

It does.
"o8" = (ad), and "98" = (us t).
"9" can be (um) and (us).
Example: her9 = herum/around and Glar9 = Glarus. Normal use.
You can't blame Google here the words are the same in other dictionaries too.
These combinations, no matter how you arrange them, always work and make sense.
Correctly written: "89 8'9 o8 98".  tum d'um ad ust

I'm not blaming Google. It's trying its best when being used for a purpose it isn't intended for.  If you give it a bit of sensible Latin, it should give you an accurate translation in English; it's got a lot better at that in recent years.  But if you give it nonsense, it will try to make sense of it even if it results in an incorrect "translation". You can test it by plugging in your phrase de unum ad ust and then changing ust to uwe, uee, urt, uyt, uut, uit, uot etc and it gives the same answer. 

You say you found the "ad ust" meaning in other dictionaries - could you please share this? My Latin is admittedly rusty but I do not know this phrase  and it does not sound Latin to me. Nor does it come up in any dictionaries I can see.  I have seen "ad alius" used occasionally in certain contexts but that is very different from yours, especially as you say ad ust is the unabbreviated version.