I'll comment on each one of them:
- There will be a session to introduce the new vegetarian products at 9:00, led by Lisa.
This sentence is great and natural. However, for a business schedule, I would recommend including "AM" to make it clear which 9:00 you're talking about.
- There will be the introduction of the new vegetarian products at 9:00, given by Lisa.
This isn't natural. First of all, you will almost never see "There will be the _____." Here, you should use "an" instead; "there will be an introduction." Second of all, this sort of passive comes off as very unnatural. It is not gramatically incorrect, but it doesn't sound right, especially in a business setting.
- Lisa will give the introduction of the new vegetarian products at 9:00.
Same thing applies here - it should be "an introduction" instead of "the introduction." There is a possible exception, however (to make things even more confusing!). If the entire event is about these vegetarian products, or if the vegetarian products are being unveiled for the first time, then it's okay to use "the." Like when Apple holds an event to unveil their new iPhone 6s, it'd be okay to say, "Tim Cook will give the introduction of the new iPhone 6." If it isn't the central focus of the event, though, then it should be "an."
- Lisa will give a speech on the introduction of the new vegetarian products at 9:00.
This one is funny for me to read, because it has a different meaning that you want it to. If someone "gives a speech on __________", then they are talking about __________. So in this case, if Lisa is giving a "speech on the introduction of the new vegetarian products," then she will be talked about the "introduction of the new vegetarian products." So maybe she will tell us about how well the introduction has been going, how well it is selling, etc. She wouldn't be INTRODUCING the new vegetarian products as you probably had meant to say.
Now down here, I'll add a note on all of these. Just because "introduction" comes from the verb "to introduce," it doesn't mean that these two can be used interchangeably, especially in a business setting. "Lisa will introduce ______" sounds much more professional and common than anything using "introduction." I would highly recommend sticking with "Lisa will introduce _______."