>Selon la légende, des spectres hantent les salles. J'ignore **ce qu**'il y a **de** vrai là-dedans, mais...

In this sentence, there are two things I cannot understand:

1. Why the need to say "j'ignore **ce que** {I don’t know **what**}" instead of "j'ignore **si** {I don’t know **if**}"?

2. I was under the impression that the phrase "il y a **du** vrai là-dedans" is a fixed expression with the meaning of "there’s **some** truth in that". So why "**de** vrai" rather than "**du** vrai"?