I never heard this word used in France. However, I would imagine people, knowing its pronunciation in English, would pronounce it like in English, except from the "r", which would be said the French way. On the other hand, people not fluent in English might pronounce it like "voocher", again with the "r" pronounced the French way.

Things might be quite different in Quebec, where this word is likely to be commonly used.

Being a native French speaker, I can say it’s pronounced as in English. That’s an English word used in French language, that’s all.

It’s not a word that I would consider as part of the French vocabulary, even with the most descriptive mindset. It could be part of the commercial aviation jargon as the only place I’ve heard it in an otherwise French sentence was in airport related contexts. The pronunciation was a pretty good approximation of an English pronunciation.

It’s difficult to find good statistics on an relatively rare loanword, so I’ll settle for multiple anecdotes as a substitute.

Voucher knew a regain of popularity in Belgium during the early Covid as it was the word used by the government to describe a mechanism to compensate travellers whose trip had to be cancelled during the lockdown.

This generated a good number of press articles and, most importantly for us, TV reportages about them. Here’s one of them (hopefully freely available to viewers outside Belgium) that exemplify the most common pronunciations of the word.

First are the ones that try to stick as close as possible to the English pronunciation as permitted by (Belgian) French phonology: [vɔwtʃœχ] by the presenter at 00:22 and [vo̞wtʃœχ] by the journalist at 01:24 (the sequence /aw/ is problematic in Belgian French and frequently replaced by /ɔw/, something similar happens to the native diphthong /wa/, that’s usually [wɑ] or rarely [wɔ]). In brackets, this gives /vɔwtʃœr/

An evolution of this pronunciation is one slightly more adapted to French phonology, by replacing the tricky diphthong by a mid high vowel. It’s the one used by the narrator at 02:58: [voˑtʃœχ̞]. In brackets, this would be /votʃœr/ (French spelling approximation: "vautcheur")

Third is a spelling pronunciation that treats "vouch-" as a French word while still recognising the suffix "-er" as coming from English, used by the interviewed couple: [vuʃœʀ̥] at 01:09 and [vuʃœχ] at 01:52. This gives /vuʃœr/, or "voucheur" without IPA.

In this second video, same pronunciations: /vɔwtʃœr/ at 02:01 by a narrator, /vuʃœr/ at 00:52 by an interviewee.

In this third video, a new variant appears: /vɔwʃœr/ by the interviewed politician at 07:28 and 08:19, again adapter to /voʃœr/ at 08:01, by the same speaker. The interviewing journalist repeats this pronunciation at 08:23.

Finally, this fourth video from a local TV channel has the narrating journalist produce /vuʃe/ at 00:45 and 02:06, clearly not realising this is loan from English.

What to conclude from all of this? We have a relatively rare loanword that still hasn’t been digested by the population at large. It is also a word that’s more common in the commercial and administrative language, which means that a lot of speakers will first encounter it in writing and not while talking to their peers. As such it is relatively vulnerable to spelling pronunciation (it looks like a native French word) and to the potential lack of knowledge of English (and the complex spelling-pronunciation rules of this language) among French speakers as a whole.

This is particularly obvious in the couple in my first video, who clearly recognised voucher as a loan from English (as shown by the pronunciation of the suffix), but didn’t know how to pronounce "vouch-" and fell back to French spelling to deduce its value. I’d interpret Reynders’s (the politician from the third video) realisation of "ch" as /ʃ/ as coming from a lack of familiarity with English too.

The second difficulty comes from the adaptation of the foreign diphthong /aʊ̞/ to French’s phonological system. In this case, it’s significant that no speaker chose /aw/ and used /ɔw/ or /o/ instead, as with almost every English (and Dutch) loan with that sound.

So how to pronounce voucher? Well, that’s open debate among French speakers right now. Come back in a few decades.