What is the unstressed vowel in marvellous?

The unstressed vowel in "marvellous" is the "u" in the second syllable, "vel".

Here's why:

* The stress falls on the first syllable, "mar".

* The "u" in "vel" is a short, unstressed vowel sound, often reduced to a schwa sound (ə).

So, the unstressed vowel in "marvellous" is the "u" in "vel".