First of all, please understand I would NEVER EVER EVER advise you or anyone to do anything I did not feel to be safe. Women have been having healthy babies without prenatal vitamins since time began and virtually all of them were eating diets so much less nutritious than ours.
I would really advise not taking the prenatal prior to getting a BFP. I'm 100% convinced it sways blue and in my experience, many of the people who included a prenatal in a pink sway had opposites. Take 1200-1600 mcg folic starting a couple months before your sway and continue through the first trimester, then gradually wean off to the amount in your prenatal. Start a prenatal and a DHA supplement when you get a BFP.
If you do the LE Diet, most of the nutrients your body needs are present in ample amounts or else are nutrients your body is very good at storing in large amounts. There are three exceptions to this - potassium, zinc, and iodine. So you need to get 2500-3500 mg potassium a day, use iodized salt for your sodium requirements when possible, and if you go for 3-6 months without getting pg, you may want to add in a low dose zinc supp. (8-15 mg) 3 days a week. I do think zinc sways blue so I don't advise this unless you've gone a long time without getting pg. If you have a tendency towards anemia, are recovering from a miscarriage or are having very heavy periods, you may want to add in an iron supp 18 mg a day if you go for 3-6 months without getting pg, but iron through supplements is not well absorbed anyway and your body is very good at both storing iron and getting whatever it can from the foods you eat (many plant-based foods have some iron in them.)