Great case Angela,
It does not look like a dermal based lesion to me.
It is a bit too deep and the vascularity is not typical of a dermal lesion.
I think the most likely diagnosis is a nerve sheath tumour.
Either a neurofibroma or schwannoma.
It has a typical shape and echo texture for one of these and also the vascularity is typical.
There branches f the medial plantar nerve pass through that anatomical location,
So I am suggesting it is a medial plantar nerve branch, nerve sheath tumour as my number 1 differential.
I have seen this before,
Ask if they have neurofibromatosis as this is a link, but even if not it can be one.
Time for an MRI scan to see if we are right!
Unfortunately a soft tissue sarcoma has to also be included on the differential list, but I think this is less likely (but not impossible).
The MRI will give the answer.
Thanks so much for posting it.
Steve.