This is mostly down to timing your death to having a ally driving the vehicles, spectating the Gunner does not count for the achievement to pop. The warthog is on most BTB game modes so makes it quite easy to achieve, since there will often be an active Warthog rolling around when you die. You don't need to spectate them for long, you toggle between teammates with
and
. The achievement will pop instantly as long as you watch the driver, not passenger or gunner.