Miss Vernon, who has used several hair dyes before, initially noticed no problems after using the Boots Colour dye.
But the following morning she woke up to find her face and eyes were red and badly swollen.
Her condition worsened over the next couple of days and when her boyfriend, Oliver Beddard, 27, returned from working away he insisted she go to hospital.
When she went to a walk-in centre her neck was so swollen nurses there were concerned she may have problems breathing, so they immediately called an ambulance.
Miss Vernon, who lives in Didsbury, Manchester, said: 'It was terrifying. I was so scared because it was really painful.
'I didn't want to go out, I didn't want anyone to see me.
'I thought if I stayed at home it would get better on its own but it just kept getting worse.'
After recovering from her injuries Miss Vernon reported the incident to Boots.
She is still recovering at home but is having to take medication and will have skin tests in hospital to find out what caused the severe reaction.
Miss Vernon, who regularly uses similar hair products, did an allergy patch test before applying it to her scalp.
She said she would be too scared to use another hair dye and other users should be made more aware of how strong the chemicals are.
Dr Chris Flower, of the Cosmetic, Toiletry and Perfumery Association, said: 'European figures show there are only around three allergic reactions for every million hair colourants used.'
He said Paraphenylenediamine (PPD) is most often the cause of these reactions and consumers should avoid it if they suffer any adverse effects.
A Boots spokesman would not reveal the dye's manufacturer as the information is 'commercially sensitive'.
The product has been on the market since 2006 and will not be withdrawn from sale as the firm viewed the incident as 'an isolated and rare occurrence'.
The spokesman reminded customers of the importance of following the instructions.