In his compositional work Vermeulen always focused his attention on melody. In his symphonies a flow of melodies can be heard from beginning to end, quite diverse in form and character. The majority of the material is asymmetrical, based on the principle of 'free declamation', that is to say: the melodic curve and length of two consecutive sentences usually vary. Frequently Vermeulen spins long melismas into ever continuous melodies, in which every memory of period structure is absent. Particularly striking is the free rhythm of flowing lines, which have become disengaged from a fixed classification of metre by antimetric figures and ties. Yet elsewhere we come across short and pithy melodies, with a clear pulsation. A characteristic feature of his music is the sophisticated climactic activity and the alternation of tension and relief, mostly supported by harmony.
In his writings Vermeulen draws a parallel between melody and the individual: "The melody is a frame of mind expressed in tones." Seen in the light of Vermeulen's line of reasoning, a multi-voiced, polymelodic composition takes on the meaning of an aural representation of society. By combining several individual melodies, he reveals the wish he cherishes for society, namely that of every individual being able to freely express and develop himself, without infringing upon other people's freedom to develop their abilities. Although Vermeulen's writings on music give the impression that he was completely consistent in applying his polymelodic concept from the beginning until the end of a piece, most of his compositions contain several passages with only one or two voices, embedded in marvellous harmonies. Open, simple textures alternate with very complex ones, as does quasi-tonality with atonal constellations.
Early on, a spirit of freedom and urge for innovation prompted Vermeulen to abandon tonality and reject the traditional form schemes. In the First Cello Sonata free atonality breaks through in spurts, which from his Second Symphony onwards determines melody and harmony in his oeuvre. As opposed to Schoenberg, Vermeulen did not choose to build a new regulatory system, but proceeded purely in terms of thematic information and its logical and psychological development. His symphonies and chamber works consequently differ greatly as far as construction is concerned. But he always succeeded in creating architectonic cohesion. The Third Symphony is in a large A-B-A form, in which A develops linearly and B is reminiscent of a Classical rondo. The Fourth Symphony is built on six themes, three of which return just before the end; the long epilogue is counterbalanced by the hammering prologue, both on the pedal tone C. The large-scale Violin Sonata is based on the major seventh, omnipresent both in melody and harmony.
Vermeulen's compositions share a unique combination of energy, power, lyricism, and tenderness. The vitality of his works is the result of the aim he had in mind: to compose as an ode to the beauty of the earth and in astonishment about life, creating music which appeals to the spirituality of man, bestowing feelings of happiness on him and making him acquainted with the source of life, the Creative Spirit. These ambitions, put into words in the book titled Princiepen der Europese muziek [Principles of European music] and numerous articles, were at right angles to the mainstream movements. Consequently, Vermeulen did not have followers or disciples.
Apart from the aesthetic-ethical 'message', which is also the subject of most of his songs, Vermeulen's symphonies and chamber music offer an ingenious interplay of melodies, a colorful sound with many felicitous instrumental ideas, fascinating sound fields, innovating parallel harmony and a captivating canon technique.