First of all let me nail my colours to the mast.
I am a bit of an opera freak/buff,call it what you will.
I have been singing since I was 8 years old,first in a choir and then after WW2 service on a semi professional basis.
NO DVD (if there is such a thing) could ever take the place of a human vocal teacher/coach.
Everyone's voice is different,and whilst a DVD MIGHT teach you the basics of singing,it could also ruin your voice and strain your vocal chords beyond repair.
You don't say how old you are,nor what sex because male and female voices require different types of coaching.
A human coach can listen to your voice see what good and bad points it has and correct them and build on what you have already.
A human coach can encourage you and make sure that your voice is set on the right path,something a disembodied DVD can NEVER do.
So to close.
A DVD for vocal coaching can be downright dangerpus,and may even rob you of any voice qualities you already have.
A human teacher can encourage and care for you voice making sure you don't strain it.
I would try and get some money together and go for a human coach.
The DVD route will only lead to straining you voice,and might even result in no voice at all,and once a voice is gone in this way there is no getting it back!