Every vowel in Siddham has two written forms, and the choice between them is mechanical. At the start of a word, where there is no consonant for a mark to attach to, the full independent letter is written. Everywhere else the vowel is a mark added to the consonant or cluster it follows, placed above, below, before or after according to which vowel it is. Nothing about the sound differs between the two forms; what differs is the position in the word.
The system rests on a further convention. A consonant sign is not a bare consonant but a consonant plus short a, so the unmarked shape is already a syllable. Writing any other vowel means replacing that inherent one with a mark, and writing no vowel at all means cancelling it with a stroke. A reader therefore parses a Siddham line into syllables rather than into letters, and a count of the characters on a page is a count of syllables.
This is what makes the script an abugida rather than an alphabet or a syllabary, and it is the property the whole Brahmic family shares. The practical consequence for anyone learning Siddham is that the forty-seven letters are the smaller half of the job. The vowel marks, the cancelling stroke, the nasal and aspiration marks and above all the conjunct forms are the rest of it, and they are where the script actually becomes difficult.