The flowers of Pachycarpus vexillaris grow in two- or three-flowered umbels from leaf axils, sessile or on peduncles of up to 12 mm long. There is a small, linear bract at the peduncle base.

Each flower has a pedicel of 8 mm to 12 mm long. The five long and thin sepals are lance-shaped and acutely pointed. In picture they are conspicuous; dark, hairy and erect around the corolla.

The five broad and long corolla lobes are ovate in shape, their pointed tips hidden by being strongly outcurved upon themselves. The lobes in picture have whitish inner surfaces suffused with pale lilac. Corollas may also be pale greenish while dull purple on the outside. The corolla is fine-textured and fleshy with glistening surface cells, hairless on both sides.

The five corona lobes arising from the base of the staminal column have long, S-shaped, three-lobed back parts, positioned in front of the sepals. The shorter, whitish front parts have pointed, lateral keels. The anther appendages are flexed in over the five-lobed style top in the centre, hard to see in the photo.

The fruit of P. vexillaris, commonly known as the mountain thickfruit, is usually a single follicle, thick as expected and spindle-shaped. The follicle is six-winged, the wings toothed. Follicle dimensions are up to 9 cm long and 2,5 cm wide in its centre (Leistner, (Ed.), 2000; JSTOR; iNaturalist).