Will's wife Terri (Jessalyn Gilsig) takes a job as the school nurse to stop him becoming closer to guidance counsellor Emma Pillsbury (Jayma Mays), and starts giving the students performance-enhancing pseudoephedrine tablets.

The episode features mash-up covers of "It's My Life" by Bon Jovi and "Confessions Part II" by Usher, and "Halo" by Beyoncé Knowles and "Walking on Sunshine" by Katrina and the Waves.

Performances by Morrison, Mays and Jane Lynch as cheerleading coach Sue Sylvester attracted praise, as did the staging of the musical mash-ups.

However, Aly Semigran of MTV and Mandi Bierly of Entertainment Weekly both noted critically that dramatic storylines in the episode dominated over the musical performances.

Sue tells Will's wife Terri Schuester (Jessalyn Gilsig) that guidance counselor Emma Pillsbury (Jayma Mays) has romantic feelings for Will.

When Kurt Hummel (Chris Colfer) tells the girls the secret behind the boys' performance, they, too, request the tablets from Terri, and give a high-spirited mash-up of "Halo" and "Walking On Sunshine".

Recurring characters who appear in "Vitamin D" are glee club members Santana Lopez (Naya Rivera), Brittany Pierce (Heather Morris), Mike Chang (Harry Shum, Jr.) and Matt Rutherford (Dijon Talton), former glee club director Sandy Ryerson (Stephen Tobolowsky), Principal Figgins (Theba), football coach Ken Tanaka (Gallagher), Terri's co-worker Howard Bamboo (Kent Avenido), and local news anchors Rod Remington (Bill A. Jones) and Andrea Carmichael (Earlene Davis).

[1] The episode features mash-up covers of "It's My Life" by Bon Jovi and "Confessions Part II" by Usher, and "Halo" by Beyoncé Knowles and "Walking on Sunshine" by Katrina and the Waves.

"[11] Aly Semigran of MTV also enjoyed the boys' performance more than the girls', and gave the episode a mostly positive review, writing that it moved the series' storylines to "a whole new level".

"[14] Anna Pickard of The Guardian called the pseudoephedrine storyline "relentlessly silly [...] but joyfully so", preferring the boys' performance to the girls' as "some excellent comedy helped me forget about Finn's dodgy autotuned vocals for once".