I just learned that recently from this video, how do words like "one" are pronounced differently from "van", the latter need to using the bottom lip to touch the upper teeth to produce the /v/ sound. Then I realise that it is much easier to pronounce a single word, compare to pronounce the word in a sentence, especially if I want to say something fast, for example "in the van were the foremost chiefs", to me it feels like I don't have enough time to touch my teeth with my lip. So my question is, how do native English speakers or linguists pronounce the /v/ sound fast in a sentence? Is there any skips during the pronunciation when speaking fast? Or I just need more practice?
Just like many other languages, (I assume) your native language doesn't contrast [v] with [w]. There are a number of languages where [v] and [w] are treated as the allophones (different realisations) of the same phoneme i.e. it doesn't matter whether you use [v] or [w], the meaning doesn't change. For example, most Indian and Pakistani English speakers use [w] for both /w/ and /v/, so they might pronounce vet and wet the same.
In English, however, those two sounds—[v] and [w]—are completely different sounds and distinguish the meaning of the word (/vet/ and /wet/ are totally different words and are distinguished by a single sound). Native speakers pronounce them easily and there's no pause while switching from /v/ to another sound (or vice versa). So if your native language doesn't have a [v] sounds, the only thing you need is practice.
[v] is made by touching your lower lip with upper teeth and by creating friction. [f] is made the same way; the only difference is voicing i.e. there's no vibration in the throat while producing [f].
When learning a new sound, in your case /v/ you begin by practising individual words. Make a list of words containing /v/. Split your list into 3 parts as follows because you will need to practise /v/ in all positions of the word:
Words beginning with v: vet van vest vase voice
Words with /v/ in the middle position: over never having reveal severe
Words ending with a /v/: have love give leave move
Practise saying your list of words aloud everyday. Gradually your lip will become more comfortable slightly pressing against your teeth.
You can then start to practise /v/ words in phrases. Here are a few to get started:
"Vegetarians and vegans have lots of veggies in their diet".
"My valuables are kept in private vault, where thieves will never find them".
"She lives in an expensive villa in the vicinity of Venice".