Many people play the Slow Vanessa with Shot Glasses paired with Lighthouse or Anglerfish, but I often found myself getting Magnus' Femur quite often and just blindly selling it because it doesn't 'belong' in the usual builds because it's not a Vanessa item.
Since I've played a lot more now, I figured why not just try it? I have to say it works a lot better than any of the Slow Burn builds that I normally (and often) play. It's probably one of the best slow payoff that you can play.
Slow Magnus Femur Vanessa | BunnyMuffins
Overview
Video Guide
Example Board












You really just need Magnus' Femur, Clamera, and Shot Glasses. Tripwire and the other slow items are purely bonuses and can be used as flex spots for anything that can support the build such as crit or to disrupt the opponent's builds such as Spyglass. Obviously Tripwire/Docklines are fantastic, but in the late game you just kill them with just Shot Glasses anyways. This particular build I won with was a Day 13 win, which is quite late for Vanessa without her Boulder/Keg builds.
Items like Jitte are great for your early game. You pair it with Grappling Hook and pretty much any slow item or slow payoff you can get such as Mantis Shrimp. Anything that says Slow on it is useful for this build.
Alternative Example Board








Here's a more typical Slow Burn Vanessa that can be used for the late game. Last slot is optional to trigger different skills. Anglerfish can be used instead of Lighthouse and saves a spot and can be used with the Friend tag in the early/mid game.
The Main Engine (Necessary Core)
The main core is just any Slow payoff with the 'giga' slow items.
Slow Payoffs: You may run 1 or more of these, but probably don't need more than 2-3 of them. Sometimes just 1 if your skills align.
- Lighthouse
- Probiscis
- Anglerfish
- Mantis Shrimp
- Jitte
- Magnus' Femur
The only realistic giga slow items are
- Shot Glasses
- Tripwire
because they just trigger the slow effect much more often than all other slow items. For example, my Docklines probably only casts once in the late game, but my Shot Glasses might get 10 instances of slow after casting twice and end the game already. It's not that the other slow items are bad, it's just not as necessary. Grappling Hook has a 7 second cooldown and the game is over before that.


Supportive Items and Skills
Any Slow Reference skill is good along with critical strike. Usually Vanessa just plays for the early game, so whatever you have from there doesn't really matter. The game I played with Femur had just 5 skills by day 13, but it's not a skill dependent build. Usually I have two full rows of skills by day 13 on Vanessa.
With the extra or 'flex spots' you can play things like Korxena Crest or any multiplier/crit item. I've done things like Card Table with Anglerfish before getting Shipwreck for example.
Lastly, slow enchants on multicast items work really well. Shipwreck with Slow enchant can make your Angler/Lighthouse go nuts.