If you always menstruate before day 30 of your cycle, you are correct that you will observe only the interval. 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.
If there is variation within the range of a few days, or even day and night of the same day, is not a veset kavua. However, if you consistently become niddah on day twenty-seven or day twenty-eight, then you should observe both days as an interval day each month unless or until you etablish one of them as a veset kavua.
Please feel free to follow up if this is unclear!