Some examples of mappings in Lua.

A better approach, instead using Snippy's mapping setup options, is to map to Lua anonymous functions that defer loading Snippy until the mapping is actually used:

local map = vim.keymap.set
map({ 'i', 's' }, '<tab>', function()
    return require('snippy').can_expand_or_advance() and '<plug>(snippy-expand-or-advance)' or '<tab>'
end, { expr = true })
map({ 'i', 's' }, '<s-tab>', function()
    return require('snippy').can_jump(-1) and '<plug>(snippy-previous)' or '<s-tab>'
end, { expr = true })
map('x', '<Tab>', '<plug>(snippy-cut-text)')
map('n', 'g<Tab>', '<plug>(snippy-cut-text)')

This is also suitable for use with plugin managers like lazy.nvim.

See the nvim-cmp wiki for an example of how to do it with cmp.

Mappings created with :vmap or mode v apply to both Visual and Select modes, which can potentially interfere with snippet expansion in Select mode. Consider using mode x for Visual-only mappings and mode s for Select-only mappings.

In Select mode, non-printable characters behave the same as in Visual mode. To make them delete the selection and enter Insert mode, you need to map them accordingly. Here is an example:

-- Make C-h, Delete and Backspace delete selection and enter Insert mode
vim.keymap.set('s', '<BS>', '<c-g>"_c')
vim.keymap.set('s', '<C-h>', '<c-g>"_c')
vim.keymap.set('s', '<Delete>', '<c-g>"_c')