You will probably need to clarify what you are asking for, as it is somewhat ambiguous. Being in a manuscript copy does not in any way prevent it from being the original text, as text is not a physical thing. So, what, exactly do you mean by Vitruvius' original writing? Is the implication that you are looking for a copy with unaltered text, or do you only want a holographic (written by the original author themselves) copy? Keep in mind that a definition of original writing being in the author's own hand would be extremely problematic in the world of Roman elites, where much of the actual writing of a fair copy would like be done by some kind of professional secretary, free or slave.

The text of De Architectura certainly still exists, but there are no surviving manuscripts of it from the first century BCE and certainly none written in Vitruvius's own hand. In fact, there are very few surviving Latin manuscripts at all from that early; one notable exception is the Qaṣr Ibrîm Papyrus, which dates to between c. 50 BCE and c. 25 CE and bears nine lines of an elegiac poem in Latin by the Roman poet Cornelius Gallus (lived c. 70 – 26 BCE). That one, though, is probably the oldest surviving manuscript bearing any portion of a literary text in Latin.

Manuscripts naturally and inevitably decay over time and are destroyed. In ancient times, literary texts were generally written on papyrus, which typically breaks down after fifty to a hundred years or so under normal conditions of storage and use. Nearly all the surviving ancient papyri come from Egypt, since the climate of basically everywhere else in the Mediterranean is too wet for them to be preserved, but the intensely dry climate of the Egyptian desert is ideal for their preservation.

Throughout antiquity and the Middle Ages until the invention of the printing press, ancient literary texts had to be copied repeatedly by hand in order to survive. Virtually all the surviving manuscripts for virtually all surviving ancient Greek and Roman texts are copies of copies of copies dating to the medieval period. As u/Cedric_Hampton notes, the earliest surviving fragments of manuscripts of Vitruvius date to the sixth century CE and the oldest surviving complete manuscript dates to the ninth century CE. This is fairly typical for an ancient Roman text.