Why won't my Vizio HDTV display In 1080p?

[dvdking21]

Well, my HDTV Is a Vizio, and It's a 42 Inch. I been having this problem for sometime though, everytime I turn on my Blu-ray player It says It displays In 720p, I always go In, and change It to 1080i because I assume that's a better looking picture than 720p. But I do try to change my resolution settings to 1080p, but It won't do nothing, and It will just say not supported. It also have 1080p24, but It's also not supported either. It just Irriates me sometimes, because I paid good money for It ( well over a thousand dollars for this back In August of 2007), and It says 1080p, but It won't display In It. Am I just doing something wrong, or Is the HDTV just don't actually have true 1080p? Well, If I can have some feedback on my problem here, I will certainly appreciate It. Thank you!