Complicated Situation

So here is the deal.

For the first time I dyed my hair (naturally dark brown) a reddish/purple/brown at a salon. Now this isn't the first time my hair has been dyed, but a first time for red. I LOVED the color. However, as many of you know, red hair fades away quite quickly and roots are especially noticeable.

So I would love to be able to refresh my red with manic panic vampire red. I have never dyed my hair at home however. My questions are:

1.) How would I go about obtaining an even color over my current color. Like I said I just want to refresh the color, but my roots are already showing. The Manic Panic directions say to apply 1/2 from the roots. But my concern is that my hair shafts are totally different from my roots.

Should I apply it heavy on my roots and use the pull through method instead?

How long would you let it sit since I just want to cover my roots, match my color, and just refresh the redness.

I want to be clear that I don't actually want the vampire red color. My hair is currently Auburn Brown with hints of purple and I like it. I just want to achieve an all over reddish color again.

The reason I want to use the manic panic is because it is $95 to get my hair dyed one color at the mall. :/ I'd rather not have to make that purchase every 1.5 months.