Dean Davies, of Old Kerry Road, Newtown, swindled more than £8,000 and tried to get a further £1,000.
The former soldier, who served two tours in Afghanistan, spent some of the cash subscribing to a porn website, perfume, iTunes, fast food and train tickets between Telford and Rhyl.
He pleaded guilty to five charges of fraud and Judge Niclas Parry told him at Mold Crown Court: “This was despicable offending.”
The 28-year-old had befriended and bullied a grandson – also vulnerable – into obtaining cards which had been entrusted to him by his grandparents.
Prosecuting, Philip Clemo said that Davies, who previously lived in Flintshire, had in 2018 befriended the grandson, who had learning difficulties, and after obtaining the cards from him withdrew cash and spent it.
The grandparents had given the cards to their grandson so he could shop for them.
Mr Clemo said on one occasion Davies had been seen on CCTV at a cashpoint. He was on benefits and when his girlfriend asked where the money was coming from he replied “mind your own f---ing business.”
The elderly grandfather, who used a wheelchair, died shortly after the offences came to light.
Mr Clemo said the grandson had told police he had become scared of Davies, and feared what would happen if he didn’t hand over the cards.
Barrister Simon Killeen, defending, said the defendant had served in Afghanistan and had PTSD and issues with depression. It was now three years since he committed the frauds.
Judge Parry, passing sentence, pointed out that the money was spent on “nothing of necessity” – and he had abused a friendship by pressure and intimidation.
Because of his previous record of dishonesty there could not be a suspended sentence.
He said the grandfather had since died but his last months must have been spent “worrying about all of this.”