This is a licencing issue, both the 100mg and the 200mg utrogestan capsules are suitable for vaginal use, but only the 200mg has a UK license for this.
Current guidelines recommend the same dose vaginally as orally, so if that's 100mg for you, then you should use the 100mg capsules rather than doubling your dose.
It can absolutely be prescribed off label for vaginal use.
However you mentioned taking 100mg orally and 100mg vaginally - whilst there's nothing wrong with this, if you don't feel great taking it orally and actually have been prescribed 200mg daily in total, you might just be better off taking the 200mg vaginally and then it won't matter if this comes as 1x200mg or 2x100mg.