Recreating the social aspects of face-to-face teaching in an online environment became more challenging in the midst of the COVID-19 pandemic. With limited or no face-to-face interaction with students, lecturers had to explore alternative ways to recreate aspects of an engaging and supportive face-to-face learning environment. This article focuses on the use of online discussion forums through the lens of the Community of Inquiry (COI) framework to meaningfully engage a mathematics community of learning. The COI framework, comprising social, cognitive and teaching presences, has been widely applied to online learning. We investigate the question: 'How can online discussion forums support the development of a learning community in a fully online Vector Calculus course?' We do this through evaluating our online discussion forum as a COI and consequently recommend design principles. Data from engineering students taking Vector Calculus in an extended degree at a South African university includes interviews, surveys, discussion forum content, and learning platform analytics. Analysis of data using the COI framework shows that discussion forums contributed to social presence, cognitive presence, and teaching presence. Design principles and directions for further research are suggested.