You say you've completed five years of regular tamoxifen, and your doctor has suggested Vagifem 10 mcg to address symptoms of dryness and itchiness. Vagifem 10 mcg is a very, very tiny dose of bioidentical estrogen, delivered as a tablet to dissolve in the vagina. I have many, many breast cancer patients who use it or other "localized estrogen" or "vaginal estrogen" options. Like you, they've had significant issues without it; over the counter creams, lubricants, and moisturizers may have had some benefit, but over time they've not done enough.
From what we know, localized estrogen doesn't enter the blood stream and get disseminated throughout your system; it is absorbed only in the genital area where it's needed. I like Vagifem because the dose is very low and there appears to be consistent absorption. But it is still estrogen, and there is sometimes reluctance to add this to a woman's regimen, especially after breast cancer.
There is a new non-estrogen treatment option for this condition. Called Osphena, it is a SERM (Selective Estrogen Receptor Modulator), the same class of medication as tamoxifen. They both target tissue and affect estrogen activity: tamoxifen targets breasts to block; Osphena targets the vagina to activate. Osphena is oral, daily, and in my practice has been well tolerated and effective. While it's been on the market for two years or so, it has not specifically been trialed in breast cancer patients (and nor have other medications, a reality I hope will change—and soon). There's not yet data on safety for women like you, but other SERMs on the market are favorable for breast health, it makes sense to think this one may be, too.
We don't have all the answers yet, unfortunately! Ultimately, the decision comes down to quality of life for you, and I'm glad it sounds like you have a health care provider who is helping you consider your options.