Vietnamese is a tone language in which the tone is a complex bundle of pitch and voice quality characteristics. The present study is restricted to Falling tones (i.e. it does not cover tone C2, called nga in Vietnamese spelling, which has medial glottalisation and ends on relatively high pitch), and deals mainly with tone C1 (hoi). This tone is generally described as falling then rising, but interestingly, some speakers realise it simply as falling. The primary aim of this paper is to investigate how these speakers maintain tone C1 distinct from two tones which are similar in terms of pitch: tones A2 (huyen) and B2 (nang). Analysis of audio and electroglottographic recordings of 7 speakers (441 syllables) confirms that there exist two types of strategies in the realisation of tone C1, and that in the falling realisation of C1, the voice quality at the end of the syllable differs from that of tones A2 and B2. It is further observed that this voice quality contrast cannot be captured by measurement of the open quotient alone, leading to general observations on the use of the open quotient in the characterisation of phenomena of glottalisation.