Which Pokémon worth still maxing out?

I'm also the same level but I'm saving all my candies/dust for my gym defenders only.

If you'd like my advice I'd leave your attackers. They are all at a reasonable CP to beat anything.
If I were to power up anything it would be the 100% Exeggutor, the Gyarados 93% (although you may find a higher IV in the future with DP it's <90%), and the Dragonite 93% (once again <90% and best defensive moveset).

Once you get some better movesets on other mons and above 90% you could consider powering them up also.

Here's my problem with Snorlax past 30. Granted, this will depend on how many you have and your local gym situation. It costs 75,000 and 66 candy to go from level 20 to 30, and 65,000 and 56 to go from 30 to 34.5. I'd rather have to level 30 Snorlax than one level 20 and one level 34.5. I find that Snorlax is good enough where I live to get a respectable gym spot, and so I needn't keep powering him up past that. That's basically my philosophy always with powering up. Level 30 unless I need higher for placement.

I think both previous comments have merit. I would also add that I use my Jolteon in almost every battle, due to the prevalence of Gyarados. Given Jolteon's lower stamina, It is one of the few attackers I plan to keep fully powered.

If you are maximizing efficiency then only power up your defenders. However, I like having a strong attacking squad for my own enjoyment, so I power up a select few just for that reason. I believe they are all worthy of investment. I do think your lowest dragonite should get those candies tho since your other two are very high as is... I only keep one dragonite for attacking so my others can go into gyms (can put the DB DP one in a gym and use the DB DC one for attacking).

Any DB Dragonite above 89-91% I would max out as far as you can go (pokemon level 39 is the current cap) for attacking. If your Vape had Aqua Tail, I would probably also power up to the max because of its versatility as attacker / defender, but with HP you're probably good leaving it be at 2825 CP.

For defense, I would continue to max out your ZH/BS Snorlax and IS Lapras all the way to the max, those guys are gonna be beasts at level 39.

I would also continue to max out your 100% Exeggutor, because that's a great moveset for attacking -- and he's basically perfect for that role.

Everything else, I don't think you should spend dust on at this point. Some very nice pokes there, but with those 5-6 pokes I mentioned above you'll be spending a lot of dust on the way to pokemon level 39.

Good luck :)

None of them 'need' further investment, but they're all worth while if you enjoy using them.

For me fun in using a Pokemon is more important than whether it's the optimum choice.

I spend dust like water, but don't mid putting in the Pidgey grind to evolve oddities like a Machamp defender - I like throwing a bit of variety around and appreciate it when I get to battle something different.

Very little is worth powering past level 30. I would say tier 1 defenders with A or B defensive move sets and tier 2 defenders with A defensive move sets are the only ones worth considering.

Well, this is my point of view, I would for sure max the 2 offensive Dragonites, the offensive Vaporeon and the Snorlax (the 93.3% with ZH-BS) and Lapras for defense.

At your level if you don't play hardcore it will take one or two months to level up to the next one, that is more than enough time to collect the dust and candies needed to power up those boys, probably around 32 dratini candies (8 per power up) and 16 Snorlax and Lapras, even using the buddy system alone you will get pretty close to them, 64 candies needed for those 4 mons (don't count the eevee ones) are 320km, and without counting you will for sure get some dratinis or other catches/hatches to help with them.

I would make Snorlax or Lapras my buddy and power up those Dragonites with the dratini candies you keep getting, there is no hurry and from my point of view the most difficult part you already have done it, getting them with high ivs and good moveset.

Also remember that if it seems a lot of candies to you after that level you will reach 34, more xp needed to next level, more time, more candies, eventually you will catch up and have all of them maxed ;)