What vitamin deficiency causes easy bruising?

Vitamin K - potassium.

Vitamin D.  this is particularly a problem for dark skinned people.  Here in Australia, The indiginous peoples who work for the govt and in uniforms indoors are given a monthly supply by the govt.  (Many are park rangers in the sunlight, so no problem). I suggest Cod Liver Oil which is a nice balance of vitamin A and D.  It takes a while to see the effect, and you should take it every day for 28 days, then try eating fish 3x a week.  In the North, you are at the end of winter, so your  body's resources are low.   As you start to get more sunshine, which is better for all humans and better absorbed you will need less intake.  Dr Mercola, a naturopath with a website: Mercola.com, is a great fan of vitamin D for health, and his site has a lot of information.

There are many reasons why a person may easily bruise.

Severe lack of vitamin C to the point of scurvy is one, but that would be unusual.

Lack of vitamin K or iron are two others

As you age bruising becomes more apparent.

There are some genetic conditions which makes a person more conducive to bruising.

The important thing is not to make assumptions about the reason behind this.

Be tested thoroughly by a medical professional.