Let me summarize some correct facts prior to answering your question:

• Vitamin D is indeed lipid-soluble vitamin, therefore it is only comes in capsules with oil or in liposomal form.
• Absorption under the tongue (sublingual absorption) is amazing for water-soluble medications, however works for lipids in lipisomal forms too.
• Any fat soluble vitamin can cause overdose in large quantities (unlike water-soluble), however therapeutic window for vitamins usually is pretty large. Therapeutic window is the dose range that can be safely taken without significant risk of overdose. Specifically for vitamin D, your recommended daily dose varies between 1000U and 5000U (depends on the season, place of living and your ethnicity). For severe deficiency we can prescribe 50000U weekly for a couple months. To overdose you need to take much more - more than 10000U daily for a few weeks.

Now to direct answer to your question - you are not missing a lot:

1. Yes with no optimal sublingual administration, absorption could differ. Bu it is not a chemotherapy, it is vitamin D - difference in health benefits if absorption dosages slightly inconsistent is minimal.
2. Whatever was not absorbed sublingually will be absorbed later in your GI tract.

Basically, don’t overthink it. And try not to ask health advice from internet strangers, better talk with your primary doctor if you have any concerns.

just get it straight from the D

To use the pharmacology vernacular, the route of administration for a medication (or supplement in this case) is a balance of pragmatic and biological considerations. Sublingual administration – aka under the tongue – gives access to a lot of capillaries, meaning the medication is quickly absorbed into the blood, without having to go through the liver and first-pass metabolism, which is what would happen with oral administration (aka swallowing a pill). As a practical matter, sublingual tends to be easier if someone cannot swallow, and because it’s very effective, the dosage can be reduced, which potentially could cost less, maybe.

I’m not a pharmacist, but you may find this video by ChubbyEmu adjacently interesting. It is about the difference in topical (aka on skin) absorption rates around the body, where the same dosage can be safe on one’s arm but hazardous on more delicate skin regions.

The absorbtion of vitamin D is greater through the capillaries under the tongue and bypass the digestive system, which is less efficient especially for those with malabsorption issues.

Just insufflate that shiz; vitamin D straight to the dome!