The factors affecting the performance of GalnP/AlGalnP vertical-cavity surface-emitting lasers (VCSELs) emitting at an attenuation minimum of PMMA plastic optical fibres (650nm) have been investigated. Using wide temperature-range and high pressure measurement techniques on equivalent (i. e the same active region) edge emitting lasers (EELs), emitting at 672nm, the temperature sensitive leakage current into the indirect X-minima is shown to be -20% of the total threshold current at room temperature. This is then estimated to rise to -70% for 655nm emission, but may be reduced to -50% by using a graded-index separate confinement heterostructure (GRINSCH). By making the same measurements on the full VCSEL structures and using a combination of thermal and gain spectrum models the performance modifying effect of the Bragg stacks have then been evaluated. It is found that that temperature dependent tuning/detuning of the gain-peak and the cavity mode is significant at low temperature due to the relatively narrow gain spectrum width. However, at room temperature and above, these VCSEL devices are shown to be dominated by the leakage current coupling to device self-heating and gain-cavity alignment is of secondary importance as the gain spectrum broadens. Similar techniques, but also including the use of a 10-band k.P based gain model developed at Surrey, have been used to investigate some of the first room temperature operational (1.3μm) GalnNAs VCSELs. The measurements support recent evidence that the material GalnNAs has an intrinsically broad gain spectrum leading to a wide temperature operating range (up to 300K) and operation at a maximum temperature of 400K. The excellent fit of measured and calculated data shows that these devices are limited by the onset of Auger recombination and also helps confirm the validity of the gain model.