I would like to think that the line, "Gather ye rosebuds, while ye may," can be read "Gather the rosebuds, while ye may. Is this reading refutable? I doubt the reading, is affirmable, but I'm hoping that the"y" could have been a substitute for thorn or the modern "th."
You would translate the sentence to modern English in the following way:
"Gather you rosebuds, while you may"
For a more verbose (and less poetic rendering) you might say: -
"You go gather rosebuds, while you are still able to"
Your sentence is actually quoted by the Oxford Dictionary and they explain the meaning of the word.
Originally ye was used only as the plural subjective form, but later its use became wider: -
"In the 13th century it came to be used in the singular, equivalent to thou. In the 15th century, when you had become the dominant subjective form, ye came to be used as an objective singular and plural (equivalent to thee and you)."
It's an exhortation to go and gather rosebuds while you are able to. Either for instance because the roses will no longer be in bloom, or the person gathering them may lose their ability to.
It would lose some of the poetic power if you replaced ye with the, as the imperative for 'you' to go gather these rosebuds (found in the first clause) would be lost.
This sense of the word "ye" is defined by oxforddictionaries.com as the
plural form of thou
The word order "gather ye" indicates that the sentence is imperative, that is, that the virgins are being "commanded" (though "urged" or "exhorted" is perhaps more accurate) to gather rosebuds while they still can.