Vaydaber vs vayomer when God instructs Moshe: what's the significance of the difference?
The torah records many places where God speaks to Moshe telling him to speak to others, generally to relay instructions. The common structure is:
... וידבר יי אל ... לאמר: ךבר
But sometimes -- I noticed this case at the beginning of Parshat Emor -- the verb is not וידבר but instead ויאמר. For example, in Vayikra 21:1:
וַיֹּאמֶר יְהוָה אֶל-מֹשֶׁה, אֱמֹר אֶל-הַכֹּהֲנִים
The next such introduction is in Vayikra 21:16, and it returns to וידבר.
What is the significance when the torah describes God speaking to Moshe using וידבר vs ויאמר ? I realize there are plenty of other cases of God speaking using ויאמר, such as many places in B'reishit when talking to the patriarchs, but maybe there's something else that makes those different.
This feels like a basic question that I ought to know the answer to, but I don't and I'm having a lot of trouble searching for an answer. Commentary, if it exists, could be attached to any of the verses, and I'm failing to get focused results from search engines.