The victim, who was in a wheelchair after a traumatic brain injury, had been approached by two women he did not know but had seen around, in Sunderland, last summer.
Newcastle Crown Court heard the man ended up in a back lane with Jamie Birrell and her accomplice, who had made the sexual proposition.
Birrell, 29, who has 116 previous convictions, then reached inside the victim's bag and stole £90, while he was being restrained by her co-accused.
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Prosecutor Paul Caulfield told the court the victim had travelled into the city by bus last June.
He said: "He was adjusting the bag he was wearing around his waist, which contained personal belongings and cash.
"He was approached by two females who he had seen in Sunderland in the past.
"He described one as the fat one, that is the co-accused, whose case has been adjourned for a pre-sentence report.
"She offered sexual favours to him and he would appear to have been willing to participate upon payment of cash.
"The three then found themselves in a back alley.
"The co-accused restrained him while Miss Birrell reached inside his bag and took from it various items, including cash in the sum of £90.
"Other items were, in fact, returned."
The court heard the women were identified through CCTV.
Birrell, of Antwerp Road, Millfield, Sunderland, who was on a community order and suspended sentence at the time, admitted theft.
Judge Penny Moreland sentenced her to 41 weeks behind bars, saying: "I think you targeted him because he was vulnerable.
"You took £90 from him, which is not a lot of money perhaps in the great scheme of things but to him would have been a very significant part of what he had available to him."
Judge Moreland said she accepted Birrell, who she said has "some potential", was "not at all proud and in fact ashamed" of what she did.
The judge added: "It seems you have begun to understand this is not the way to spend your life."
Jennifer Coxon, defending, said Birrell, who sent an apology letter to the court, has reconciled with her family and has support.