I'm so glad that someone mentioned Lanturn, as I imagine I'm one of very few people to be using it in the new meta. Lanturn is actually a very effective check to Thundurus-T and Tornadus-T, as you surmised. With SpDef investment, it avoids being 3HKOed by LO Hurricane, and 2HKOed by LO Focus Blast. It is also capable of OHKOing both Therians (in the Rain). However, I've found that Lanturn is not a great check to Rain as a whole. While I initially thought it would be a great Keldeo check, the only Keldeo set it checks is Specs. The reason for this is that Keldeo is able to OHKO Lanturn after SR with Secret Sword (I forget whether or not it needs an LO). However, Lanturn can switch in on Specs Hydro Pump with defensive investment, and proceed to Thunder the pony's butt off.
Lanturn seems to be set-up fodder for Breloom and a free switch-in for things like Latios, but I've found that people switch into Lanturn too rashly. Perhaps it's because of Lanturn's RU status, but everyone seems to forget it has access to Ice Beam. Lanturn is able to OHKO 4/0 Breloom after SR with an unboosted Ice Beam, and the switch is so obvious in most battles. Even if you miss the switch-in, most people foolishly try to set up on Lanturn, and so lose their Breloom on that turn. Latios is 2HKOed, even without SR. In short, always run Lanturn with Ice Beam.
I would actually recommend against using Lanturn on a defensive team. Unlike ScarfWynaut, I do not run a defensive team so I'm no expert, but such teams are often hard-pressed for team slots, to be able to handle every role that needs to be filled. With six Pokemon, you have to be able to achieve many roles on a defensive team, and each Pokemon must contribute. But Lanturn doesn't really DO any certain thing on a defensive team. You're forced to wonder why you're using it over Vaporeon who can Wishpass, or Jellicent who can Spinblock, or even sometimes Starmie who can Spin. On defensive teams, it's not enough to check a few dangerous threats, because you have to help fulfill one of the necessary roles on that team, which Lanturn simply cannot do. Yes, Lanturn gets Heal Bell, but Celebi and Chansey/Blissey actually make better clerics on a defensive team, because they have recovery (and SR if that's needed).
I've found that Lanturn actually does best on Bulky Offense or offensively-leaning Balanced teams. It isn't fast enough and can't boost its SpAtk (outside of the unreliable Charge Beam) to fit on an HO team, but the slower offense routes fit it perfectly. Keep in mind, Lanturn does not wall very much of anything. It is an offensive check to threats, and should be treated as such. Lanturn works best when you can pivot it around and wait and get the kills you need it to, which is pretty much exactly what Bulky Offense and Balanced teams do. This is where its mix of skills allow it to use Heal Bell in a way that no other cleric can. I prefer to use it with Agility though, because once Ferrothorn is down, it cleans up Rain teams pretty dang easily. But even a SubCharge Beam set is viable, since it can out-speeds almost all walls and it only takes 52 EVs to make 101 HP Subs. Additionally, it can OHKO most of its offensive checks at +1. Tbh, I think Agility is more suited to the metagame than SubCharge Beam though. The point is, Lanturn can actually run several sets, more than you would think at first.
Oh, and don't worry about Lanturn's stats. It actually has a trolly speed, just beating CB Scizor without any investment. Specially, its HP is massive enough to give it good bulk, although invetment in SpDef is necessary for it to switch into the two Rain Therians. And while its SpAtk seems low, keep in mind it'll be spamming 120 bp STAB attacks most of the time, and that they'll be super-effective against the things its checking. Lanturn is just fine specially, despite how its stats look. Just don't try and take more than one physical hit per game.
I would disagree about Lanturn and Vaporeon. First of all, 11/17 types are able to hit them both at least neutrally, so I don't see what you mean by "they only don't resist Grass". So their type-synergy is quite poor, although their role-synergy is much better. Secondly, I've already given my feelings about how Lanturn does on defensive teams, but suffice to say I don't think that uses Lanturn to its full potential.
But perhaps the biggest problem I have here is your set. Can you please explain to me in better detail what you were thinking? Of course Lanturn becomes set-up fodder with that set. First of all, it lacks Ice Beam, which I've already elaborated on, but let's forget that for a second. Lanturn isn't a wall, it just doesn't have the stats or movepool for it. What job is it performing on your team that couldn't be done better by another Pokemon? Hell, even your own Vaporeon could probably serve better as a cleric than Lanturn. Lanturn can't just take repeated hits, like a defensive Pokemon needs to be able to, it can only take enough hits to check the threats it checks. Which your Lanturn doesn't do. It doesn't beat Thundurus-T and Tornadus-T. In fact, it debatably even loses to Starmie. Volt Switch, with your EV spread, doesn't even 2HKO Tornadus-T most of the time. That's right, not even a 2HKO. It can just keep switching around to stay healthy via Regenerator the whole match. Not to mention that Volt Switch makes you send in another Pokemon, so I hope you have another Hurricane resist. Thundurus-T can Nasty Plot on your weak Scald and then 2HKO you with Focus Blast. Or it can Nasty Plot twice, and then OHKO you with Focus Blast. Yes, it can take two Scalds (even in Rain). So I guess I'm wondering how exactly your Lanturn beats them. Oh, and why give it Toxic? Lanturn doesn't have any way of Toxic-stalling (Protect/recovery), and it lacks an effective way of spreading Toxic around (phazing).
I don't mean to sound rude, but I really think you're misusing Lanturn. Your set doesn't beat the things Lanturn is supposed to beat, and it doesn't have the tools to do the job you want it to do. I feel like your team slot would be better occupied by another Pokemon. Don't get me wrong, Lanturn is an awesome Pokemon, but not in that way. It's not a defensive wall, it's an offensive pivot.
