Hey Kev, yeah, it's too soon to think of taking in another kitty, but when the time comes, you'll know it. And a thought for you about that: Vicky won't be thinking you want to replace her -- she'll know that the love you shared was just so great that you had to have the company of another of her species. She will know, too, that this will give you happiness -- something she can no longer do for you, but wants for you more than anything. All in good time. Hang in there!
I've just seen this thread, I am so sorry for your loss, but think Vicki was pretty lucky to have had these past 4 years with you, and to know she was loved even when she was so aggressive when unwell.
Please also accept my sympathies on the loss of your step-father.