Is it feasible for a first rank Explorator to survive in a vacuum by having the best version of bionic respiratory? I was thinking that would be a neat hook for how you survived being ship lorn.

Well, maybe. If the exposure to vacuum is short enough, but you have to deal with the other effects of vacuum (aside from suffocation): the pressure differential will cause simultaneous boiling and freezing of any uncovered sources of moisture (with the attendant soft tissue damage) like eyes, nose, mouth/throat, probably ears. Also, count on the same to any open wounds, and other moisture sources (ie, if it's sweaty, you do not want it exposed to death pressure), and pretty impressive bruises forming on exposed skin, possibly haemorrhaging through the pores. Oh, and expect some rather impressive and unpleasant outgassing from the digestive tract, which will help act to chill the relevant orifice, and better hope there's no air trapped in tooth cavities.
And that doesn't consider the increase in radiation exposure, or the fact that eventually, they will need some more air to breathe, as well as food/water.

That said, a character with best-quality bionic respiratory system will retain a sufficiently oxygenated bloodstream to remain usefully conscious for longer, which obviously increases their chances of reaching a shuttle or saviour pod. And if they have a sealed suit of carapace or power armour, all-encompassing bodyglove (including a hood), or a void suit, then the effects of vacuum are either negated or mitigated, meaning the limiting factor before reaching a better stocked place of safety is the stored air (6 hours in a standard navy void suit, or just under one watch), although the bionic lungs would extend that so the limit is how long before they pass out of starvation or dehydration.

So, plausible if it's a/the reason they managed to reach a shuttle/saviour pod.
Also plausible if it's the result of them getting caught in death pressure on the way there (aside from eyes, the throat and lungs are going to suffer the greatest soft-tissue damage from vacuum and/or explosive decompression).

Ah thanks. So The Flesh of Weak is really the only non suit way to survive in a vacuum thanks to the Machine trait?

Alasseo said:

I'd expect the Voidskin implant to take care of most of the problems that the respirator doesn't handle.

Chuckachu said:

For an Explorator, it's the most accessible method. For non Explorators, there is the Augmentacist alternate career path that allows Physical Perfection (also giving the Machine Trait).