Copyright question about illustrating photographs


Let's say, hypothetically that I am illustrating the photograph of an iconic person and have obtained permission to use material from one of their descendants of some musician to use material from them in a design, but the photo is owned by some else. I'm not using the photograph, but drawing a vectorised image from it. Do I need to obtain copyright permission from the copyright holder of the photograph then, or not, or is it a grey area?
I would say that if you are using the photograph as a guide you are therefore creating original art and should be OK.
Legally, I've no idea .. sorry. But in my opinion, I'd say if it was a one off photo that someone has taken / bought and owns personally, the original photograph is property of that person and not the person in the picture. So I'd be inclined to ask their permission to make an illustration out of it before the descendant of the person in it. I'm sure someone will correct me if I'm wrong here.
I would say that if you are using the photograph as a guide you are therefore creating original art and should be OK.

Hope so. I need to know for sure.

I have the permission to use photography of the person from the copyright holder of his material (his son).

The person is now dead and I think also the the photographer is dead also, not sure. Can't find them anywhere on the internet.

So if both the person and the photographer area dead, where do I stand? It's a complicated busiess this. I thought I'd gained full use of copyright material but now I am not sure...
Just thinking - if you were to draw/paint a portrait of someone from a photo the artwork that you have created is yours, nothing to do with the photographer. All you are doing is using a computer to do the same. If the subject and the photographer are dead (if it's over 50 years it will likely to be out of copyright anyway) who is going to come back at you? Make it your own so it is not exactly as the photo and who will know?
The person in question died more recently than that. If this goes to print it will be put on display in a commercial venue, and the vectorised images will be identifiable as produces from this particular photo. No-one will probably know, but regardless I need it to be on 100% solid ground so far as copyright law goes.
Was hoping for some free advice here. In any case I have contacted someone who knows the photographer and is in the process of contacting them about. They also said he is old and in poor health, so this could be a problem. I don't. It's a complicated business. I am carrying on with it anyway and hopefully the copyright issue will be resolved favourably and it wont all be for nothing. Thanks to al for the advice.
I got in touch with the photographer and have been given copyright permission. Had a nice chat with them as well about the person in question.
Maybe search for a microstock image of this person. purchase a print license and go nuts in illustrator?.