[Fizzlemizz]

local vendors = {
    [Tuukanit] = {Distilled Fish Juice, Improvised Sushi},
    [Elder Nappa] = {Distilled Fish Juice, Improvised Sushi}
}

Each key should be a string and each entry table (optional for the last) needs a comma at the end:

local vendors = {
    ["Tuukanit"] = { ["Distilled Fish Juice"] = true, ["Improvised Sushi"] = true, },
    ["Elder Nappa"] = { ["Distilled Fish Juice"] = true, ["Improvised Sushi"] = true, },
    ["Fred Vendor"] = { ["Distilled Fish Juice"] = true, ["Improvised Sushi"] = true, },
    ["Samantha Goods"] = { ["Distilled Fish Juice"] = true, ["Improvised Sushi"] = true, },
    ["Trade Ya]" = { ["Distilled Fish Juice"] = true, ["Improvised Sushi"] = true, },
}

if you just added the item string into the table as values it would be created as a number ordered table eg.

local vendors = {
    ["Tuukanit"] = { "Distilled Fish Juice", "Improvised Sushi"], },
    ["Elder Nappa"] = { "Distilled Fish Juice", "Improvised Sushi", },
    ["Fred Vendor"] = { "Distilled Fish Juice", "Improvised Sushi", },
    ["Samantha Goods"] = { "Distilled Fish Juice", "Improvised Sushi", },
    ["Trade Ya]" = { "Distilled Fish Juice", "Improvised Sushi", },
}

is the same as:

local vendors = {
    ["Tuukanit"] = { [1] = "Distilled Fish Juice", [2] = "Improvised Sushi"], },
    ["Elder Nappa"] = { [1] = "Distilled Fish Juice", [2] = "Improvised Sushi", },
    ["Fred Vendor"] = { [1] = "Distilled Fish Juice", [2] = "Improvised Sushi", },
    ["Samantha Goods"] = { [1] = "Distilled Fish Juice", [2] = "Improvised Sushi", },
    ["Trade Ya]" = { [1] = "Distilled Fish Juice", [2] = "Improvised Sushi", },
}