yes there is no exact number of days you need to be in the uk to qualify for ILR as a spouse (unlike other types of visas) but remember the UK is suppose to be her home and its at the discretion of the case worker if you've been outside the UK for a long period whether to grant you ILR.

but if you have a good reason , as you said your job is outside the UK and your wife joins you then her chances of getting should increase if she had been outside the UK holidaying

when it is your wives time to apply for ILR, if the case worker is not happy your wife has spent 10 months outside the UK, they might tell her to apply for FLR instead

but yes i've heard of a few people who have been granted ILR while been out of the UK for long periods.

evidence supply your contract of employment, air tickets stubs, bills in Vietnam, pics etc to show you both were together,,