If the law ( leaving the E.U on the 29th March) doesn’t apply to the result of the vote..?

What vote? You're not making any sense.

It would mean the the will of the people in the UK doesn't matter and that the UK is not a real form of democracy anymore. The British people voted to leave the EU. British leaders need to respect that and faithfully make it a reality.

The date is based on two things -- an Article of the EU treaty and a statute adopted when the UK opted to withdraw from the EU. While there are reasons why neither is 100% binding, the ultimate reality on any legal rule is that it ultimately comes down to a court or bureaucratic agency to enforce it. Since, here, the legal rule applies to the legislature and the executive, it will come down to the courts to enforce it, and the courts might find a reason to avoid ruling on this issue.

The treaty part is simply an agreement between the countries that can be modified or waived with the consent of all of the parties.

The statute, in theory, would require another act by Parliament to change it. That statute may be binding on the UK government but it is not binding on the EU.

Putting aside the symbolic value of saying that the UK is no longer on the EU, I am not sure that there is any immediate practical effect. During the time that the UK was part of the EU, due to the treaty obligations, a good chunk of EU law was written into UK statutes and will, therefore, remain part of UK law until those statutes are changed. Thus, the immediate effect of withdrawal will be that UK courts will not have to comply with the orders of some European Courts on some matters. Once withdrawal is complete, the UK and EU will have to establish border controls (which, given the Good Friday Accords is why the borders of Northern Ireland have been such a big issue). Neither side is actually ready for border controls which is why those issues are postponed for later determination under the current draft of the withdrawal agreement which has not been approved yet by parliament and might never be approved. And that's why both sides might ignore the statute unless or until a court rules that the UK is out of the EU.

If the law ( leaving the E.U on the 29th March) doesn’t apply to the result of the vote... can we assume that the laws of the land are also not applicable?