The fillers in vagifem can sometimes irritate and if you're really dry they don't always dissolve fast enough. If Estriol is working for you then just use that, it's trial and error where dosage is concerned. It might have been stopping the Vagifem that's caused the breast pain with a drop in estrogen, who knows.
When I first started Vagifem on the loading dose every day for two weeks I got pelvic cramps and nausea in the morning. When I then went down to twice a week I got the breast pain. Twice a week was never enough and gradually over the years the dose has increased to every day now. I'd definately continue using Estriol every day and just add more if you need to. You can always reduce the dose if you feel comfortable doing so, everyone is different and like CLKD said the leaflet is out of date and needs rewriting to reflect the difference to systemic HRT, the safety and extremely low dosage.