If you always menstruate before day 30 of your cycle, you are correct that you will observe only the veset haflagah. This is not because you have a veset kavua, but because you get your period before reaching your other veset days. If you ever do get your period later, you will keep yom hachodesh and onah beinonit like any woman who does not have a veset kavua.

Everything you wrote shows a correct understanding of the concept of veset kavua. A woman with a veset kavua gets her period either at regular intervals or on regular dates on the Hebrew calendar. This veset needs to be precise not just to the date, but to the daytime or night time onah.

If there is variation within the range of a few days, or even day and night of the same day, your cycle would not be considered a veset kavua. However, if you consistently become niddah on day twenty-seven or day twenty-eight, then you should observe the single onah of your veset haflagah according to your last period, and on your other typical onot you should check before relations by wiping externally with a tissue or toilet paper, not right after urinating, and abstain if you find any bleeding.

Some poskim are stricter with a woman who has a consistent pattern in which there is a 2 or 3 day window when her period begins, and consider the entire 2 or 3 days an extended veset haflagah. Rabbi Kenneth Auman, the posek of the site, holds that relations are permitted on the additional days beyond the single onah& of the veset haflagah, so long as there is no sign that the period is imminent.

Please feel free to follow up if this is unclear!