Brightlances on Vypers are not a good choice in my view at all, since they are far too inaccurate, and cost too many points.
As for the other weapon options, there are several schools of thought:
Shuriken Cannon: This has the advantage of keeping the Vyper cheap, and is pretty handy for anti-infantry duty, and blowing up vehicles with weak rear armour. The problem is you have to get within rapid-fire range to be able to use it, and this can be dangerous for Vypers, as their fragility means that even mass Bolter fire can damage them. If you are fielding a lot of Jetbikes and Vypers, and you want to be aggressive with them, then this armament is probably okay, but if you are looking for more of a fire support role from the Vyper, then this is not the best choice in my view.
Scatter Laser: This is identical in terms of its use to the Shuriken Cannon, but has the advantage of a longer range, allowing it to keep out of the way of those pesky rapid fire weapons. It is also still quite cheap, so if you have the points, it's probably a better bet than the Shuriken Cannon.
Starcannon: This is not a cheap option, and the Starcannon suffers from the improvements to cover save in fifth edition, although, when mounted on a skimmer, you can try to manoeuvre in such a way as to expose units to its firepower from angles whereby they are not protected by cover. This is, however, rather fiddly, and can expose the Vyper to too much return fire if you are not careful, so I do not recommend taking this weapon, unless you know you are going to face Deathwing, or an army list chock full of Terminators.
EML: The greatest advantage of this weapon is its range. If you want a Vyper which can hang back and provide long range fire support, then this is the option to go for. That said, it's horribly inaccurate, and it isn't that cheap, so there will be games where you may find that your Vypers do not achieve that much as a result. It's an unreliable choice in my view, but it may appeal to you if you are looking for support fire
Shuriken Cannon upgrade: This can be useful when combined with the EML firing Plasma Missiles if you want your Vyper to focus on anti-infantry duty. The problem, however, is that you have to get within rapid-fire range to make the most of this additional firepower (see earlier comments about the downsides to this). I don't recommend taking this upgrade with any other weapons, due to the defensive weapon rules.
I hope that helps.
