what is the best vegan substitute for lard?
The best substitute in general for lard would be a combination of vegetable shortening - which is generally hydrogenated palm oil - and butter.

Since this is a vegan substitute, you'll have to just stick to the shortening.  That's OK - shortening was quite literally invented to replace lard - but you'll lose a lot of the flakiness and full flavour if you don't mix it with some butter.

For pastry crusts, you might also mix some coconut oil in with the shortening; the flaky texture essentially comes from saturated fats, for which coconut oil ranks pretty highly.  Just don't use too much, as it imparts a not-so-subtle flavour of its own.