With Leadership values being particularly important in Warhammer 30k due to the chance of routing and fleeing off the board, Legion Vexilla are important to take for helping to counter these chances - especially for large squads. Indeed, for my 20 marine strong tactical squad, I will be taking a vexilla along with a few other upgrades.
To model the Vexilla, there are a number of options. Forge World supply parts that attach to the top of the back pack of the space marines. But I was a little bit dissatisfied with this approach. If one examines the front cover of some of the Black Library books (in particular the cover of Sedition's Gate) it is clear that the Alpha Legion has their vexilla carried by marines rather than on their backs. I much prefer this kind of approach since it feels more authentic - to inspire the troops, the legion symbol and history can be hoisted high by the bearer.
For this miniature, I therefore too a chaos space marine icon pole and attached a space marine banner to the top of it. The pose of the miniature certainly suggests that the marine is proudly displaying the banner front and centre and rallying the troops around him against the xenos or other enemy. The conversion is finished off with a bolter conversion, plus parts from space marines, chaos space marines, and grey knights. As with a number of my miniatures for my Alpha Legion army, the legs will be added later once I've converted them to true scale ones.