You can use anydruid ring to travel to the Vesspyr Isles, but must have unlocked access to use the one nearby in Obol Plains. This applies to portal spells cast by players and the guild hall amenity too.
Alternately, use the Dragon Circle in the Great Divide if you are under level adventure 92 and have not completed the access quest for Obol Plains.
About locating the willows and shrubs:
Most of the willows look like smaller, curved versions of the trees on the islands and can be found near the outer edge of each island. With caution you can easily gather these even if you are well below the level of the monsters.
The dead tree is similar in shape and size to all of the other willows; it just lack leaves.
The shrub is found on the island with the giant flowers and it often spawns just beneath the giant leaves of the vines that cover the outer edges.
Provided you have chosen it, you can use the "Track Harvestables" ability from the Tradeskill Alternate Advancement tree to help find them. Only one will be up on each island at any given moment.
Alternately, you can use Tab on your keyboard to target those which can be gathered, which is quite easy when flying on the outer edges of the islands.
They have up to a 45 second respawn time before they reappear in the same location, so flying around to locate the next one is advised.
About crafting the candle
A drakota fuel merchant is in the building if you forgot to bring fuel and a broker is nearby outside if you need to buy materials.
1 iridium cluster
1 panther meat
2 plump haddock
2 cutthroat trout
5 Effulgent candles
All of the trees can be harvested as often as you like, at any time, and placed in your housing or sold on the broker.
The candle recipe is a permanent addition to your recipe book and can also be placed in housing or sold on the broker.
If you're too lazy to return and gather more willows later, the Frosty Willow is also available for purchase using Loyalty Point Tokens.